Support for .NET Standard

[khionu]

Most packages are going the way of .NET Standard, if they can, and it would allow for better support for ASP.NET Core projects.

[argentini]

UPVOTE!

[trullock]

I'm happy to try and do this, but I need some build instructions.

The nested submodule make this very complicated to even know where to begin

[trullock]

I've made a crudely hacked together netstandard2.0 compatible version here:
https://www.nuget.org/packages/libsassnetstandard/3.3.7

I'll happily pull this nuget if we can get this repos up to date