[settings] Update plugin to add settings and features

• Add playlist callback support.  You can now optionally instruct your
  transmitter to be enabled/disabled when a playlist starts/stops

• Configure whether we use RT, RT+, or simply PS for RDS output.  The
  user can test which works with the RDS equipment they care about and
  select the one that works for them.  RT+ is the "best" one since it
  sets the actual fields for artist/title.

• Configuration page now has a little more info and updates the bug list

TODO:

• Help in the form of tooltips or something user-friendly to explain
  RT/RT+/PS and other settings that may be confusing to the user.