Moved hooke.playlist -> hooke.plugin.playlist and added hooke.plugin.Builtin.

Moved hooke.playlist -> hooke.plugin.playlist and added hooke.plugin.Builtin.

Highlights:
  * Cleaner Playlist, with more consistent file format.
  * New "Builtin" class for required (core) commands.
  * Moved NotRecognized from hooke.driver -> hooke.curve to avoid
    import-order issues.  Now hooke.curve can be fully loaded before
    hooke.driver.
  * Moved hooke.curve.Data.notes -> hooke.curve.Data.info['note']
    for better consistency with Curve and Playlist.
  * Started cleaning up plugin loading into flexible functions.