Resume creation of partial playlists

[aag]

Currently, if the script detects that a playlist already exists, it assumes the playlist is complete and skips it. Instead, we should verify that the playlist has the full number of songs expected for that chart and if not, pick up where the last run left off.

This will probably require changing the create_playlist_from_chart() function to accept a number of songs, so we know how many to expect.