Broken markdown file extension regexp

Hello!

On the last edition of the the regexp that matches markdown file extensions, the part that matches the beginning of the file extension of removed. I was setting up a site with some [Rmarkdown](http://rmarkdown.rstudio.com/) files, and `.Rmd` files were being processed as regular markdown files. Patched!

I specifically used `.Rmd` in the test. I hope that is ok! I used this fix with an [Rmarkdown converter](https://github.com/atbradley/jekyll-rmarkdown), and it worked fine (but out of scope of this PR).

Cheers!