Add .markdown to file extention list

Brett,

Long time listener to your podcast, and thank you for all you do in the community.  I Found nvremind the other day and couldn't be more thrilled - this will come in very handy!

Except I prefer `.markdown` to `.md`. So I fixed nvremind! Here are those changes!

If there's anything I need to do to polish it up some more please let me know! Rock on!