Add matplotlib theme for creating graphs for newsposts and wiki articles

[bdach]

In the past, I have been responsible for creating graph illustrations for multiple performance points newsposts. I've had the styling code lying around on my machine, and people have been asking for it now and again - I've used gists in the past to spread this around, but that doesn't seem sustainable anymore what with multiple pp committees and multiple contributors.

I wanted to clean this stuff up and put it in a git repo, but I wasn't sure what was the best place for it - @Walavouchey suggested that the osu! wiki may be able to take them in under the meta/ directory. So, here they are, PR'd.

If you think there's a better place to put this than the wiki then I'm all ears. Originally I was thinking maybe a separate repo under the ppy org, but I'm not sure about visibility in that case.

README.md contains an explanation how to use these. It's autogenerated from the README.ipynb jupyter notebook. Ideally I would commit just the notebook and not have to bother with the generation, but github doesn't like it - the notebook will render just fine if you open the particular file, but won't render if you open the containing directory itself. Therefore the extra steps with the generate_readme.sh script.