Fix blinking progress bar on song change.

The progress bar and status line sometimes blink in between songs.
For a demonstration, see this video: http://youtu.be/-Z4ZLjJc1Z4
Fixes :addall error in #45.
#43 Change default markers.
#46 Add date to format options.
#38 Include disc numbers and disc sorting in library.
Update help file with new features.
Optimize :addall for speed.
Merge branch 'master' of github.com:boysetsfrog/vimpc
Add catch all for deleting a playlist

This will prevent the function trying to delete a playlist that doesn't
exist and erroring.