Implement Go to Definition for API docs

The Go to Definition feature generates links to GitHub source for class members and module functions e.g. 

https://github.com/tpetricek/FSharp.Formatting/blob/master/tests/FSharp.MetadataFormat.Tests/files/FsLib/Library1.fs#L15-15

Right now it accepts a pair: source repository and source folder where the code is compiled to DLL. We assume that two sources have the same directory structure.

I added a test to show how the feature should be used.