Support .NET Standard 2.0

[crskycode]

I've created a branch with .NET Standard 2.0 support (based on master). Would it be possible for you to create a new branch to merge these changes?

If you’re also able to publish a NuGet package based on this branch, that would be incredibly helpful. Thank you so much for your time and consideration!

[Nominom]

Hi!

Thanks for adding support for 2.0. I created a PR based on your changes that multi-targets the nuget package for both 2.1 and 2.0 now.

I'll do some additional testing still, and maybe clean up the test projects a little.

I'll upload a new nuget version once that's done.

[crskycode]

Thank you so much!