Transform GitHub repo directory/file link references into rich snippets

Transform GitHub repo directory/file link references into rich snippets with popouts; similar to the issue/PR links gitterHQ/gitter#000

Input:

https://github.com/gitterHQ/gitter/blob/master/README.md

Should transform into:

gitterHQ/gitter#master/README.md

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Same with commits, Input:

https://github.com/gitterHQ/gitter/blob/c731915cdbac9a05373013b1da835045c449c9e5/README.md

->

gitterHQ/gitter#c731915/README.md

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Please note that this enhancment should not trample linking text, etc with markdown unlike the current issue/PR links where they just show the rich snippet instead of just linking the text.

Here is [this file](https://github.com/gitterHQ/gitter/blob/master/README.md)

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Related issues with auto-linking and referencing:

• Reference issue comment
• Better auto-linking error message in private chats (was: doesn't work for pull requests)
• Easy reference to another room
• No trample on inline text links when using references

Hi @MadLittleMods, great writeup. Technically, adding new link handlers should be fairly easy. See here: https://github.com/gitterHQ/gitter-markdown-processor/blob/master/lib/github-url-matcher.js.

Let's discuss it and perhaps it's something you would be interested in doing as task?