Expand zero plugin to read any given tag

### Problem

The zero plugin, [per documentation](https://beets.readthedocs.io/en/stable/plugins/zero.html?highlight=zero), should erase every tag that isn't listed under `keep_fields:`. However, on my end, it keeps a bunch of tags regardless.

-vv doesn't really tell me anything beyond listing a bunch of fields like this:

```
$ beet -vv zero mew
user configuration: /Users/redacted/.config/beets/config.yaml
data directory: /Users/redacted/.config/beets
plugin paths:
Sending event: pluginload
artresizer: method is (2, (7, 0, 10), False)
library database: /Users/redacted/.config/beets/library.db
library directory: /Volumes/Expansion-1/Media/Audio/Sound
Sending event: library_opened
zero: genres: -> None
zero: genre: -> None
zero: lyricist: -> None
zero: composer_sort: -> None
zero: arranger: -> None
zero: grouping: -> None
zero: comments: -> None
zero: bpm: 0 -> None
zero: comp: 0 -> None
zero: artist_sort: -> None
zero: albumartist_sort: -> None
zero: asin: -> None
zero: catalognum: -> None
zero: barcode: -> None
zero: disctitle: -> None
zero: encoder: -> None
zero: script: -> None
zero: language: -> None
zero: country: -> None
zero: albumstatus: -> None
zero: media: -> None
zero: albumdisambig: -> None
zero: date: -> None
zero: month: 0 -> None
zero: day: 0 -> None
zero: original_date: -> None
zero: original_year: 0 -> None
zero: original_month: 0 -> None
zero: original_day: 0 -> None
zero: artist_credit: -> None
zero: albumartist_credit: -> None
zero: art: -> None
zero: mb_trackid: -> None
zero: mb_releasetrackid: -> None
zero: mb_workid: -> None
zero: mb_albumid: -> None
zero: mb_artistid: -> None
zero: mb_albumartistid: -> None
zero: mb_releasegroupid: -> None
zero: rg_track_gain: None -> None
zero: rg_album_gain: None -> None
zero: rg_track_peak: None -> None
zero: rg_album_peak: None -> None
zero: r128_track_gain: None -> None
zero: r128_album_gain: None -> None
zero: initial_key: None -> None
Sending event: write
zero: genres: None -> None
zero: genre: None -> None
zero: lyricist: None -> None
zero: composer_sort: None -> None
zero: arranger: None -> None
zero: grouping: None -> None
zero: comments: None -> None
zero: bpm: None -> None
zero: comp: None -> None
zero: artist_sort: None -> None
zero: albumartist_sort: None -> None
zero: asin: None -> None
zero: catalognum: None -> None
zero: barcode: None -> None
zero: disctitle: None -> None
zero: encoder: None -> None
zero: script: None -> None
zero: language: None -> None
zero: country: None -> None
zero: albumstatus: None -> None
zero: media: None -> None
zero: albumdisambig: None -> None
zero: date: None -> None
zero: month: None -> None
zero: day: None -> None
zero: original_date: None -> None
zero: original_year: None -> None
zero: original_month: None -> None
zero: original_day: None -> None
zero: artist_credit: None -> None
zero: albumartist_credit: None -> None
zero: art: None -> None
zero: mb_trackid: None -> None
zero: mb_releasetrackid: None -> None
zero: mb_workid: None -> None
zero: mb_albumid: None -> None
zero: mb_artistid: None -> None
zero: mb_albumartistid: None -> None
zero: mb_releasegroupid: None -> None
zero: rg_track_gain: None -> None
zero: rg_album_gain: None -> None
zero: rg_track_peak: None -> None
zero: rg_album_peak: None -> None
zero: r128_track_gain: None -> None
zero: r128_album_gain: None -> None
zero: initial_key: None -> None
Sending event: after_write
Sending event: database_change
```

I noticed this because my imported collection has all sorts of garbage tags that I didn't list on `keep_fields:`. I've tested importing an album many times with different `zero` settings each time, and in all cases there was a `tool` tag I created to see what would happen to it, and it's still there.

Maybe the way zero works doesn't really take care of these custom tags that many programs can create and instead goes through a list obvious things like `genre` or `comment` where it indeed works and removes them.

### Setup

* OS: macOS High Sierra
* Python version: 2.7.16
* beets version: 1.4.9 and 1.5.0 work the same

My configuration (output of `beet config`) is:

```yaml
directory: /Volumes/Expansion-1/Media/Audio/Sound/

plugins: convert discogs duplicates fetchart embedart lyrics missing zero

autotag: yes

art_filename: cover

clutter: .pdf .bmp .png .jpg .jpef .tif .gif .sfv .nfo .m3u .log .DS_Store Thumbs.DB Thumbs.db

original_date: yes

languages: en

va_name: Various

ignored_media: .vob .mp4 .mkv .avi .ts .pdf .cbz .cbr

import:
copy: no
write: yes
move: yes

paths:
default: $albumartist/$album%aunique{albumartist album, tracktotal albumtype year label,()}/$track. $title
comp: Various/$album%aunique{albumartist album, tracktotal albumtype year label,()}/$track. $title
albumartist:various artists: Various/$album%aunique{albumartist album, tracktotal albumtype year label,()}/$track. $title
replace:
'[\\/]': _
'^\.': _
'[\x00-\x1f]': _
'[<>:"\?\*\|]': _
'\.$': _
'\s+$': ''
'^\s+': ''
'^-': _
convert:
tmpdir: /Volumes/Expansion-1/Media/Audio/Sound/
auto: yes
delete_originals: yes
never_convert_lossy_files: yes
format: mp3
mp3:
command: ffmpeg -i $source -q:a 0 -map_metadata 0 $dest
extension: mp3
discogs:
user_token: redacted
duplicates:
album: yes
move: /Users/redacted/Downloads/
fetchart:
auto: yes
sources: fanarttv itunes amazon coverart albumart google
fanarttv_key: redacted
google_API_key: redacted
embedart:
auto: yes
maxwidth: 1000
lyrics:
auto: no
force: yes
sources: genius musixmatch lyricwiki google
google_API_key: redacted
fallback: ''
import.write: yes
replaygain:
auto: no
backend: command
chroma:
auto: no
zero:
auto: yes
fields: ''
keep_fields:
- albumtype
- artist
- title
- album
- year
- composer
- albumartist
- track
- tracktotal
- disc
- disctotal
- label
- lyrics
- rating
- acoustid_fingerprint
- acoustid_id
- first played
- last played
- play count
- images
update_database: true
```

I have also tried:

removing `fields: ''`
using `fields: []`
using `keep_fields: albumtype artist title album year composer albumartist track tracktotal disc disctotal label lyrics rating acoustid_fingerprint acoustid_id first_played last_played play_count images`

It is also unclear to me how to list fields without an underscore such as `first played` (not `first_played`) with this plugin. Because the listing is one string, it would recognized as `first` and `played` separately. My solution is includes these `-` listing, which I must've took from somewhere, and I don't know if they're right in the first place.