Add "before_choose_candidate" event
Add "before_choose_candidate" event

* 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.

beetbox/beets,Freso/beets,Kraymer/beets,jackwilsdon/beets,beetbox/beets,Kraymer/beets,parapente/beets,Freso/beets,Freso/beets,xsteadfastx/beets,SusannaMaria/beets,jcoady9/beets,diego-plan9/beets,madmouser1/beets,mosesfistos1/beetbox,LordSputnik/beets,madmouser1/beets,pkess/beets,MyTunesFreeMusic/privacy-policy,pkess/beets,mried/beets,artemutin/beets,shamangeorge/beets,SusannaMaria/beets,xsteadfastx/beets,lengtche/beets,Freso/beets,SusannaMaria/beets,mried/beets,jcoady9/beets,jackwilsdon/beets,jcoady9/beets,jackwilsdon/beets,mosesfistos1/beetbox,mried/beets,sampsyo/beets,lengtche/beets,MyTunesFreeMusic/privacy-policy,ibmibmibm/beets,LordSputnik/beets,Kraymer/beets,shamangeorge/beets,diego-plan9/beets,xsteadfastx/beets,sampsyo/beets,ibmibmibm/beets,Kraymer/beets,MyTunesFreeMusic/privacy-policy,beetbox/beets,pkess/beets,ibmibmibm/beets,jcoady9/beets,xsteadfastx/beets,lengtche/beets,parapente/beets,swt30/beets,swt30/beets,lengtche/beets,pkess/beets,swt30/beets,artemutin/beets,ibmibmibm/beets,madmouser1/beets,LordSputnik/beets,diego-plan9/beets,mried/beets,SusannaMaria/beets,jackwilsdon/beets,sampsyo/beets,madmouser1/beets,sampsyo/beets,shamangeorge/beets,mosesfistos1/beetbox,shamangeorge/beets,parapente/beets,LordSputnik/beets,swt30/beets,parapente/beets,MyTunesFreeMusic/privacy-policy,artemutin/beets,mosesfistos1/beetbox,beetbox/beets,artemutin/beets,diego-plan9/beets