Add support for dynamic UTIs.

Support dynamic UTIs that appear in Mavericks if no Markdown app has been installed:

* dyn.ah62d4rv4ge8043a - md
* dyn.ah62d4rv4ge80445e - mkd
* dyn.ah62d4rv4ge8042pwrrwg875s - markdown
* dyn.ah62d4rv4ge8045pe - mmd

(I determined those by creating files with the extensions, then running `mdls` on the files.)