`beet modify`: Treat arguments with : before = as queries, not modifications

This can be useful if the value being queried contains an equal sign.

Sample shell session with the problem:
```
$ beet -c $PWD/config.yaml ls -f '$path'
/dev/shm/beets-bugs/beep.ogg
/dev/shm/beets-bugs/beep=beep.ogg
$ beet -c $PWD/config.yaml modify 'path:/dev/shm/beets-bugs/beep=beep.ogg' artist=nobody
Modifying 2 items.
 -  - beep
  artist:  -> nobody
  path:/dev/shm/beets-bugs/beep: beep.ogg
 -  - beep=beep
  artist:  -> nobody
  path:/dev/shm/beets-bugs/beep: beep.ogg
Really modify (Y/n)?
```