POPM (popularity meter) tag support

[jmbannon]

POPM seems to be the standard tag used for ratings across many music apps. This PR adds support for this tag.

Closes #23

[jmbannon]

Summoning @sampsyo @JOJ0 for a review 🙂

[JOJ0]

Hi @jmbannon sorry for a little late reply. Basically I think it's about time this tag gets integrated in mediafile. Great!

maybe you can put as much context into the PR description as possible, for documentation purposes.
I'm actually not sure if this repo has it's own changelog and how it's documented, so my only idea is to put as much as possible in the PR's description. I know you linked to the issue, but maybe copy over everything that is really relevant/true/worth knowing about his change. Which programs use this tag (eg Traktor I read, what else?), link to the docs on id3.org and so on.

And then there is something @Wolkenarchitect mentioned which I don't entirely understand: #23 (comment)
Is that relevant? AFAIS you implemented a basic tag that is a an integer? Is that enough?

[JOJ0]

Ah, found it, there actually is a changelog: https://mediafile.readthedocs.io/en/latest/#changelog
and this is the source: https://github.com/beetbox/mediafile/edit/master/docs/index.rst

But it seems like @sampsyo did additions to that log in the past when pushing out a release. So not sure if you should add to it within this PR