* Add '-m', '--musicbrainzid' option to the import command, allowing the user
to specify a single MusicBrainz ID to be used for the candidate lookup instead
of trying to find suitable candidates.
* Modify lookup_candidates() of ImportTask and SingletonImportTask to use the
musicbrainz id if present.

* Modify the "--musicbrainzid" argument to the importer so multiple IDs can be
specified by the user instead of a single one.
* Revise autotag.match.tag_album and autotag.match.tag_item signature to expect
a list of IDs (search_ids) instead of a single one (search_id), and add logic
for handling and returning multiple matches for those IDs.
* Update calls to those functions in other parts of the code.

* Add tests for the "--musicbrainzid" argument (one/several ids for matching
an album/singleton; direct test on task.lookup_candidates() for
album/singleton).

* Replace the entities used on ImportMusicBrainzIdTest mocking the calls to
musicbrainzngs.get_release_by_id and musicbrainzngs.get_recording_by_id instead
of querying MusicBrainz.
* Other cleanup and docstring fixes.

* Fix an issue that caused the candidates for a singleton not to be
returned ordered by distance from autotag.match.tag_item(), when
searching multiple MusicBrainz ids (ie. several "--musicbrainzid"
arguments). The candidates are now explicitely reordered before being
returned and before the recommendation is computed.
* Fix test_importer.mocked_get_recording_by_id so that the artist is
nested properly (and as a result, taken into account into the distance
calculations).

* Style cleanup and fixes for the "--musicbrainzid" import argument.
* Allow the input of several IDs (separated by spaces) on the "enter Id"
importer prompt.
* Add basic documentation.

* Store the user-supplied MusicBrainz IDs (via the "--musicbrainzid"
importer argument) on ImporTask.task.musicbrainz_ids during the
lookup_candidates() pipeline stage.
* Update test cases to reflect the changes.

* Rename the importer argument and related variables to make it more
generic, as the feature should be independent of the backend used and
not restricted to MusicBrainz.
* Update documentation and docstrings accordingly.
* Add changelog entry.