Add GitHub Flavored Markdown mode, Add some more Markdown fileExtensions

Note you can only ever get into GFM mode using the Language Switcher.
fileExtensions source: https://github.com/github/markup#markups
Change name, remove blockComment
Merge pull request #9780 from MarcelGerber/gfm-mode

Add GitHub Flavored Markdown mode, Add some more Markdown fileExtensions