Add generic boolean commands

This adds a helper for boolean commands, and moves Pause, Random and
Repeat to use it. It also adds new methods for "consume" and "single"
Add relative and absolute seeking

This adds Seekrel and Seekcur which allow seeking the current song
either relative to the current position, or to a specific time
respectively.
Add additional playlist related commands

This adds PlaylistFind, PlaylistId, PlaylistSearch and PlaylistChanges