Add Go To Definition for Node.js require() calls

You can now press F12 on a `require()` call and WE will open the resolved source file in the preview tab.

This works for _any_ `require()` call, except for Node.js built-in modules.
I could add an option to let the user specify the path to these modules (which are not shipped as source with Node.js binaries) and make them work too.

@madskristensen, how can I make this support [Peek Definition](http://msdn.microsoft.com/en-us/library/vstudio/dn160178)?