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
:+1: on the "GitHub Markdown" naming. Another alternative is "GFM", but I would frown upon that because not everyone may know of it.