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
AFAIK GFM has the exact same file extensions Markdown has, and I'm not sure what happens if two modes use the same file extensions.
You can still use GFM using the Language Switcher, where you can define it as default mode as well.