* Fixes for documentation of the prompt event (missing reserved letter, typos).
* Update sample mbsubmit plugin to conform to the new PromptChoice structure.
* Update mbsubmit plugin in order to use PromptChoice instead of the previous
ExtraChoice.
* Remove comments as they are now present on the pull request and documentation.
* 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).
