Add the ability to download a youtube playlist.

The dapl and dvpl commands take playlist URL or ID as paramters and
download all the audio or video tracks in the playlist respectively. It
creates a subdirectory in the default download directory named the same
as the playlist name. It then downloads all the tracks into the
subdirectory.
Remove repeated code (#110)

Replace invalid characters in subdirectory name
Merge branch 'pr/110' into develop