Allow setting the description of the output playlist

[jerboa88]

Users may want to specify the description of the output playlist.

Tasks:

• Add an argument --description. If no description is provided, a default one will be used that included the script name and the last updated timestamp of the playlist
• Add an argument --description-mode with 3 options:
  • replace (default) - Replace the output playlist description with the provided one
  • append - Append the provided description to the existing output playlist description
• Parse the provided description using strftime so that date format codes can be used