Diff output shows formatted values for strings

This is why, in #807, the album field was appearing to "work" while the year
field was displaying the issue. We were colorizing the wrong value.

Now, of course, we need to fix the underlying issue: we're confusingly
displaying album-level information for these items.
Show file path when deleting items

Using `beets remove -d <target>" now shows the file path when asking for
confirmation.

The format string is '$path - $title'.

Fix issue #320.
Zero plugin: do not zero important fields

Prevent zeroing fields id, album_id and path.

Fix issue #475.
Merge pull request #811 from brunal/zero-conserve-important-fields

Zero plugin: do not zero important fields
Changelog for #811 (fix #475)
Merge pull request #810 from brunal/show-path-on-delete

Show file path when deleting items
Changelog for #810 (fix #320)
Also check whether new fields are in the restricted field list (Fix #812)
Make beet write not show item names for items without changes.
Changelog for #812
Fix #822: scrub/convert plugin conflict

We now correctly scrub the destination of a conversion, not the source.
Fix broken docs references