Refactor playlist parsing

[hasezoey]

This PR updates the playlist parsing (m3u, pls, asx, xspf):

• completely re-implement pls parsing
• remove asx related code from xspf
• update xspf to use options over empty strings
• update asx to use options over empty strings
• update asx to be a little more spec compliant (refuse entries without at least one title and ref href)
• move the tests to their respective modules
• add more tests to each module
• actually test the detection and redirection of the main playlist decode
• rename all PlaylistItem to their respective playlists name
• some slight cleanup (removing unused functions, improving code style, doc-comments / comments)