Add explicit .net standard 2.0 support

[mungojam]

Please add this such that there are no additional dependencies listed on nuget when bringing into a .net standard library. I think it will get rid of some of my build warnings too but not certain.

[jamesmontemagno]

There are no additional dependencies as I specify them for each platform. if you install into a .net standard library you need it to have the 1.6 installed.