Support user defined lyric retrieval

This pull request adds the support for users defining their own Artist and Track to get the lyrics for, so it does not have to be playing.  This will enable other addons (Like the Sonos Addon) to make use of the work already done to grab lyrics.

Discussed in the thread:
http://forum.kodi.tv/showthread.php?tid=147340&pid=1841636#pid1841636