Add support for 8tracks.

I've added support for 8tracks as requested in  #18. Based off of the Songza plugin.

One thing that I did notice: it doesn't look like the "percent" Track attribute is used anywhere anymore. As far as I was able to trace, the usage of "percent" went away way back in 75c0c6ad6a (when the `pollSongInfo` method was rewritten during your refactor to plugins) and never came back. Was this on purpose?

I've tested my changes and the extension does scrobble tracks when finished, though the "Now listening to" status disappears after a few seconds.