First attempt mixed project support

Following the disussion in fsproject/ProjectScaffold#101 I did this spike for supporting C# projects better.

This is still just single project,  but:

* C# projects (probably not Winforms I think, but normal class libs and XAML) are now supported with regards to renaming and content replacement in the init.fsx file
* Added Cs proj + assemblyinfo.cs files to the sourcelink target.
* init.fsx is now prepared a bit for any multi project support that might or might not come in the future, I've tried to reduce the number of hardcoded names so make this easier.

Tested with C# main project and F# test project.