* Update test for extra fields in edited yaml, allowing the user to add fields
while editing. Rename the test to test_single_edit_add_field to reflect its
purpose more accurately.
* Update unit tests in order to reflect the changes on the last refactor of
edit.py (patch edit.edit instead of EditPlugin.get_editor, revise stdin strings
to match current version, remove TODO on docstrings from malformed and invalid
yaml tests).
* Add tests for executing the command with album queries (-a), testing the edit
of album and albumartist fields.
* Revise invalid and malformed yaml tests so they return a failure instead of
an error.
* 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.
