play: New option: --end-of-playlist.

play: New option: --end-of-playlist.

If the end of the playlist is reached, para_play starts over with the
first file of the playlist. This commit provides the --end-of-playlist
option to control the behaviour. The new option has three pre-defined
values: stop, loop, quit with "loop" being the default. Since lopsub
features multi-valued options, the implementation is pretty simple.