zero: Can delete embedded images

[geigerzaehler]

Fixes #1100.

I also propose we add the media file fields to the output of beet fields. Currently the documentation for the zero plugin refers to the ouput of beet fields for a list of tags that can be zeroed. This is not quite correct as not all fields are written to the tags.

[sampsyo]

Again, this looks great, and thanks for including the tests. I'll merge this now.

You're right that the pointer in the docs is indeed incorrect, modifying the beet fields output does seem like the right solution. To make the output most intelligible, maybe we should avoid repetition since the fields are so similar for the two sets? For example, we could use a --tags flag or somesuch to explicitly request the MediaFile fields instead of the Item fields, or we could mark the MF fields in the default list somehow (e.g., with an asterisk *).

[jgeboski]

Kudos!