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
"GitHub Flavored Markdown" is much longer than all of the other language names, so the dropdown list is now much wider. What about just "GitHub Markdown"?