* Add example "mbsubmit" plugin, which allows the user to print the tracklist
of an unmatched album during the import process in a format understood by
MusicBrainz track parser.
* The plugin listens for the before_choose_candidate event and appends the
track printing options depending on the task status (candidates and
recommendation).
* Add "before_choose_candidate" event, sent to the plugins during
TerminalImportSession.choose_match(). This event allows plugins to append
additional choice to the user prompt during importer.
* The event is sent on the main loop of choose_match(), with session and task
parameters. The plugins are responsible for checking the task status in order
to append choices in a sensible way.
* Add "ExtraChoice" namedtuple to improve readability and encapsulate the
concept of a single extra choice.
* Add "format_raw_length" global configuration option to allow the user to
toggle between human-readable (default) or raw formatting of track durations.
* Modify library.Item in order to have length formatted as H:MM instead of the
raw number of seconds by using a types.Float subclass (DurationType).
* Add library.DurationType, with custom format() and parse() methods that
handle the conversion.
* Add dbcore.query.DurationQuery as a NumericQuery subclass that _convert()s
the ranges specified by the user to floats, delegating the rest of the
functionality in the parent NumericQuery class.
* Add ui.raw_seconds_short() as the reverse of human_seconds_short(). This
function uses a regular expression in order to allow any number of minutes, and
always required SS to have two digits.
* Make emitters produce a pair (dict, Item), in order to preserve the output
at print_data (dict is used if no custom format is specified, Item otherwise).
* Simplify the handling of the paths, printed at the top of print_data. The
path key is removed from the dict entirely and fetched from the Item.
