Add .Net Standard 2.0 target

[vhatsura]

According to Open-source library guidance it would be good to target .Net Standard 2.0 to benefit from having a smaller package graph.

I would appreciate to help with it.

[vhatsura]

@nblumhardt , where are you planning to release stable version?

[nblumhardt]

Hi @vhatsura - I've opened #78 to track, it probably just needs a quick check over by eye, and smoke-test of the -pre package; any feedback on that ticket would help. Cheers!