Add .markdown to file extention list

[rwilcox]

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!