scripts: add CPU usage with colorization

[vd: add a note about sysstat, update the CHANGELOG, add a min_width and
no separator to the cpu block]

Closes #11
scripts: (cpu) rename and comment out

This commit renames the "cpu" script to "load_average", as suggested by
@deimosfr. With the previously added "cpu_usage" script, "load_average"
becomes not really relevant, so let's comment it out for the moment.