zero: Permit stripping album art

[jgeboski]

It would be nice to have the option of automatically clearing embedded art when an item is imported. Whether or not a media item actually contains embedded art, beets should ensure the resulting media item has no embedded art after being import. There are two plugins which would offer a good place of implementation for this feature: the EmbedArt and the Zero plugins.

The EmbedArt plugin already supports a command called clearart which allows for the manual stripping of embedded art from items which match a query. Since the the clearart operation is not automatic and there is no option for automation, an extra step is required on the importation of media.

What probably makes more sense is implementing support for the art field in the Zero plugin. It can only be assumed that people who would use such a feature already have the Zero plugin deployed for clearing other fields. That said, it would require less configuration as all a user would need to do is drop the art field in their configuration for the Zero plugin. Moreover, with the EmbedArt plugin, it embeds art into media items by default. This feature would need to be disabled in the configuration as well.

[sampsyo]

Seems quite reasonable. This would take the form of a special case in the zero plugin that would need access to the underlying MediaFile (and so would perhaps need to use a different plugin event).