scripts: (cpu_usage) flatten the script

This commit flattens the cpu_usage script, and add a few tweaks, like:
using $! instead of $?, setting default values for warning and critical
thresholds, set the urgent flag if critical value is reached, and
redirect stderr to stdout.

The interval is also increased to 10 seconds.

Refs: #11
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]