UI modify command: ability to delete/reset an attribute

Through this PR, I would like to discuss the UI syntax for deleting/resetting an attribute.
This is a need that was already discussed in #530.

For the moment, I have implemented something simple:
```beet modify album:'In Rainbows' pl_kids=!```

When using ! as an attribute value, it will call ```del obj['attr']```.

An alternative could have been:
```beet modify album:'In Rainbows' !pl_kids```

NB: the tilde ~ character (instead of !) does not seem a good idea, as it is interpreted by the shell.

My need is: I set tags (i.e. flexible attributes) on items/albums to build smart playlists (e.g. ```beet modify album:'In Rainbows' pl_work=1```). Sometimes I change my mind and remove some of these tags. But I don't want them to stay forever in the DB (e.g. after ```beet modify album:'In Rainbows' pl_work=0```). So I need a way to remove a flexible attribute.