Support for wiki links

[fgblomqvist]

Since GFM is used on the GitHub wikis, it would be appropriate to add support for the wikilink syntax (https://help.github.com/articles/adding-links-to-wikis/):
[[Link Text|NameOfYourWikiPage]]

I'm not sure if this works in MD files across GitHub, my guess is it doesn't. But nonetheless, the only negative aspect of adding is that someone might try to use it, sees that it looks good in the GFM render, and then pushes it, just to realize it's the wrong syntax.

Another way of doing it could be to check for a specific file, e.g. .wiki in the root folder of the project, and if it has it, enable the syntax. I think this way is pretty elegant (and theoretically, it would also allow you to do the same kind of broken-link-checking that the github page talks about).