badfiles: config options/command for fixing files

How about a separate config entry for commands to fix bad files instead of just listing them. Also one could possibly run a separate command (for example `beet fix` or `beet bad -f` ) to fix every file in the library instead of just checking.

Suggested config structure:
```
badfiles:
check:
mp3: mp3val
...
fix:
mp3: mp3val -f -nb
...
```

Start by locating the existing badfiles configuration and command entry point that currently only checks files. Review how format-specific check commands are loaded and how a library-wide operation would be invoked. Done means the proposed separate fix commands and an explicit way to apply them across the library are defined and covered by appropriate behavior checks.

Written by the indexing model from the issue text.