Major overhaul with a lot of new features and old bugs fixed

Hi there,

I had some time to go over my old version of the script and ended up changing quite a lot:

- fixed bugs
  - Spotify stopping when Ad plays (#3)
  - Exiting the script kills all running instances of mpv
  - Exiting script while local playback is active leaves Spotify muted
  - Script will launch multiple times when opening Spotify URIs through a browser
  - AdKiller does not start if Spotify takes to long too launch (race condition)
  - Script will mute unrelated applications under some circumstances

- new features
  - automatically choose music player
      - supports mpv, vlc, mplayer, mpg321, avplay, ffplay
      - can be overridden with a custom music player setting
  - choose music player volume
  - automatically choose default Music folder based on XDG specifications

- various improvements
  - change ALERT to freedesktop standard
  - a lot more comments
  - fixed typos

- changes to the documentation
  - added section on dependencies
  - expanded section on installation and made it more generic
  - added section on settings
  - added important notes
  - added trade mark statement

I'd appreciate it if you took a look at the commits. Please feel free to leave me a comment if there's something you would like to change.

Cheers,
Feltzer