Create an extensible music player

[squigjess]

• JukeBot currently supports fetching audio streams from YouTube.
• Local file playing and Spotify link translation will come down the line.
• An extensions/plug-in system is being brainstormed currently.
• To prepare for both, moving the music player (i.e. the !play command) from "YouTube by default" to "local files by default, extensible via plugins" will both allow for easier implementation of the extensions system and will allow us to move the YouTube player functionality to an extension...
• ...thereby further separating JukeBot from other music bots that play YT audio by default (thereby protecting the main JukeBot project in the event of a love letter from YT).

[squigjess]

With Spotify integration now an (optional) part of the main package (see e0fb0a9), this is no longer really necessary.