convert: WavPack can be either lossless or lossy

### Problem

When `convert: never_convert_lossy_files` is set to `yes`, all WavPack files are copied instead of converted regardless of their losslessness, because [currently `convert` treat `wv` files as lossy](https://github.com/beetbox/beets/blob/5f26ec6db9a852f623ea4dfeb2f99f7b43bb819b/beetsplug/convert.py#L46). It seems [current logic](https://github.com/beetbox/beets/blob/5f26ec6db9a852f623ea4dfeb2f99f7b43bb819b/beetsplug/convert.py#L104) is not sufficient to determine losslessness of WavPack files.

According to http://www.wavpack.com/wavpack_doc.html#wvunpack, `wvunpack -f9` can be used to obtain the encoding mode of a `wv` file. Can we handle `wv` files correctly by checking [`MODE_LOSSLESS`](https://github.com/dbry/WavPack/blob/76eab1023836f53501c0e3bc5d1702e62a3330c5/include/wavpack.h#L322) and [`MODE_HYBRID`](https://github.com/dbry/WavPack/blob/76eab1023836f53501c0e3bc5d1702e62a3330c5/include/wavpack.h#L323) of it?

### Setup

* OS: Ubuntu 16.04.3 LTS (on Windows)
* Python version: 3.6.4
* beets version: 1.4.6

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

```yaml
directory: ~/Music/Beets/Library
library: ~/Music/Beets/library.db
per_disc_numbering: yes

paths:
default: $album%aunique{}/%if{$disctotal-1,,$disc-}$track $title
comp: $album%aunique{}/%if{$disctotal-1,,$disc-}$track $title

ui:
color: yes

import:
write: yes
copy: yes
bell: yes

plugins: absubmit chroma convert embedart export fetchart fromfilename fuzzy info inline mbsubmit mbsync missing random web
absubmit:
auto: no
extractor: ~/bin/abzsubmit-0.1/streaming_extractor_music
chroma:
auto: no
acoustid:
apikey: REDACTED
convert:
auto: yes
never_convert_lossy_files: yes
format: alac
formats:
auto: ~/.config/beets/convert.py $source $dest
mp3: ffmpeg -i $source -y -vn -aq 1 $dest
wavpack:
command: wavpack -h $source -o $dest
extension: wv
aac:
command: ffmpeg -i $source -y -vn -acodec aac -aq 1 $dest
extension: m4a
alac:
command: ffmpeg -i $source -y -vn -acodec alac $dest
extension: m4a
flac: ffmpeg -i $source -y -vn -acodec flac $dest
opus: ffmpeg -i $source -y -vn -acodec libopus -ab 96k $dest
ogg: ffmpeg -i $source -y -vn -acodec libvorbis -aq 3 $dest
wma: ffmpeg -i $source -y -vn -acodec wmav2 -vn $dest
dest:
pretend: no
threads: 4
max_bitrate: 500
tmpdir:
quiet: no
embed: yes

paths: {}
no_convert: ''
copy_album_art: no
album_art_maxwidth: 0
embedart:
auto: yes
remove_art_file: yes
maxwidth: 0
compare_threshold: 0
ifempty: no
fetchart:
auto: yes
sources: coverart filesystem amazon albumart
store_source: yes
minwidth: 0
maxwidth: 0
enforce_ratio: no
cautious: no
cover_names:
- cover
- front
- art
- album
- folder
google_key: REDACTED
google_engine: 001442825323518660753:hrh5ch1gjzm
fanarttv_key: REDACTED
web:
host: 127.0.0.1
port: 8337
cors: ''
reverse_proxy: no
include_paths: no
mbsubmit:
format: $track. $title - $artist ($length)
threshold: medium
pathfields: {}
item_fields: {}
album_fields: {}
fuzzy:
prefix: '~'
threshold: 0.7
export:
default_format: json
json:
formatting:
ensure_ascii: no
indent: 4
separators: [',', ': ']
sort_keys: yes
missing:
count: no
total: no
album: no
```

The conversion decision is in beetsplug/convert.py around lines 46 and 104; first inspect that logic and run wvunpack -f9 on representative WavPack files to observe the encoding mode. Done means never_convert_lossy_files distinguishes lossless and lossy WavPack files correctly instead of treating every .wv file the same.

Written by the indexing model from the issue text.