* Modify existing tests in order to explicitely check for differences (or lack
of) in the items fields, with the intention to ensure that no unintended changes
slip through.
* Added tests for modifying a single item from a list of items, and for editing
the album field of the items (stub for discussing whether the actual album
should be updated, etc).
* Add tests for the edit plugin, including a helper for bypassing the manual
yaml editing. Tests are focused on "black-box" functionality, running the
command and making assertions on the changes made to the library.
