Source Link support

Let's add [Source Link](https://github.com/dotnet/sourcelink) support to this package. I'm unsure whether this is something that only can be enabled for .NET Core projects or if it's supported for .NET Framework as well.

A blog post explaining the functionality can be found [here](https://www.hanselman.com/blog/ExploringNETCoresSourceLinkSteppingIntoTheSourceCodeOfNuGetPackagesYouDontOwn.aspx).