scripts: (mediaplayer) new script

Add the mediaplayer script for displaying "[ARTIST] - [TITLE]" in a
block if this information is available for a running media player.

This script requires playerctl, available here:

https://github.com/acrisci/playerctl

Playerctl works with all media players that implement the MPRIS DBus
Interface Specification:

http://specifications.freedesktop.org/mpris-spec/latest/

Closes #19

[vd: move the block near the volume one and comment it out, as playerctl
is unfortunately not officially packaged yet on common distribution]