Add netstandard2.0 target framework.

[ChrisRobbers]

Proposed Changes

This addresses #428, by adding a target framework for netstandard2.0 in addition to netstandard1.5. The extra nuget dependencies required for netstandard1.5 can simply be omitted.

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[michaelklishin]

Per discussion in #428 there seem to be no reason not to add a new target.

[michaelklishin]

Thank you!