Add ability to specify fields with `beet write`

### Use case

I'm trying to use beets to correct some tags on the music files

### Solution
The library already contains the correct tags for the entire album, but I want only to reflect some of them on the music files, for instance the year of the track. Running `beet write` will write all tags in the library, not just the track's year. I'd like to have a `--fields` flag that allows me to specify which tags to write:

$ beet write "All the Things She Said" --fields year,artist_sort

On the other way around, suppose I have the correct field in the music file, but the release wasn't properly on MusicBrainz, and I like to have the correct value on my beets' database; I'd also like to have the flag `--fields` on the `update` command to specify which fields beets should overwrite on DB from file:

$ beet update "All the Things She Said" --fields year,artist_sort