Add more matplotlib lessons

I added a few basic lessons in #177, but there's lots more material on Rougier's site. The regular plots section looks like a good one to do next; it even has before and after code. There are also some more sources listed in issue #177. Anyone who wants to contribute lessons can find more instructions in CONTRIBUTING.md. Create a new issue with a brief description of the lesson you want to write, and comment that you want to write it.

It might make sense to add navigation through the lessons to the browser or PyCharm before adding more lessons.