Add GitHub Flavored Markdown mode, Add some more Markdown fileExtensions

Note you can only ever get into GFM mode using the Language Switcher.
Also added some more `fileExtensions` for Markdown. Source: https://github.com/github/markup#markups
How about "Markdown (GitHub)"?  That way the two related options will be next to each other in the list, and easier to discover.  (As it is, this doesn't feel like the greatest workflow, but I agree it's better than nothing).