Add support for dynamic UTIs.

I think this might solve the problem for those in issue #29 that were not resolved by PR #25. This also fixes the bug I reported in #37. Dynamic UTIs are created (at least in Mavericks) for files with extensions that no application has registered.

I added support for these file extensions: md, mkd, markdown, mmd. I determined the dynamic UTIs by creating files with the extensions, then running `mdls` on the files.