simplify playlists and update usage

-allow setting a named playlist when one is already active
  backup the current one first
-pull set-active-playlist to a sub
-make get/set playlist name return 'main' when no named pl is set
add zero-argument playlist command to print the active playlist
rename the file with the current playlist name

.klomplist-name => .klomp-playlist
avoids conflict with backed-up playlist dirs
use separate history files for klomp playlists