update project to target multiple frameworks

[quezlatch]

so we can still use the nuget package if we're not on core...

[ForNeVeR]

Could you please update the publishing docs so we could know how to publish the multi-target package to NuGet?

[quezlatch]

exactly the same command will work, though you will need those frameworks installed which probably means compiling from windows

[rspeele]

Works for me. Thanks!