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
Since GitHub Flavored Markdown is a subset of markdown, I'm not sure if we can/should list any extensions: if we do, they would have to be the same as the (regular) markdown extensions, but that could create conflicts inside Brackets. I'm not sure, but perhaps just leaving it to the user to switch to GitHub Flavored Markdown mode is the best option.