mdp 0.91.2 (new formula)

mdp is "A command-line based markdown presentation tool." This program was
announced recently on Hacker News and has proved useful.

(Some fixes pending https://github.com/visit1985/mdp/issues/35)
Can this test be modified to do something more substantial then e.g. `--version` or `--help`? See `cmake.rb` for an example of a formula with a good test. Thanks!

I'd suggest running this on the `sample.md` you install.