Provide a more natural volume percentage with ALSA.

Provide a more natural volume percentage with ALSA.

The rationale of the code is explained in the header:
http://git.alsa-project.org/?p=alsa-utils.git;a=blob;f=alsamixer/volume_mapping.c;h=1c0d7c45e6686239464e1b0bbc8983ea57f3914f;hb=HEAD
> The mapping is designed so that the position in the interval is
> proportional to the volume as a human ear would perceive it (i.e., the
> position is the cubic root of the linear sample multiplication
> factor).

and the commit message:
http://git.alsa-project.org/?p=alsa-utils.git;a=commit;h=34bb514b5fd1d6f91ba9a7b3a70b0ea0c6014250
> use a mapping where the bar height is proportional to the audible
> volume, i.e., where the amplitude is the cube of the bar height.

and further explanation can be found in the pull request:
https://github.com/i3/i3status/pull/268#pullrequestreview-147429763