Add Mediawiki support for README files

[bryceac]

I have quite a few projects here on GitHub, and I like using the Mediawiki markup language over Markdown simply because it is easier.

However, I have noticed on the crates that I publish that HTML code, particularly ones used for formatting purposes, like definition lists and ordered lists, is displayed on the crate page itself.

While I can do some conversion, thanks to a cargo crate I found, to help me get closer to how it should look, it would be nice if Markdown was not the only supported markup language, much like how GitHub supports both Markdown and Mediawiki’s wiki text.