Add support for Guayadeque and mpv

I love this script, by the way, very useful. I thought I'd add an option to install the mpv media player (fork of mplayer2, see page here: https://github.com/mpv-player/mpv and https://github.com/mpv-player/mpv/blob/master/DOCS/man/en/changes.rst) and the Guayadeque music player (http://guayadeque.org). Might want to look it over, I'm not exactly a command line/github master, I just edited the script.