Add netstandard2.0 target framework

[thomasgalliker]

Do you think it's a good idea to add netstandard2.0 to the list of target frameworks in the csproj file?

Replace TargetFramework tag with TargetFrameworks:
<TargetFramework>netstandard1.0</TargetFramework>
becomes...
<TargetFrameworks>netstandard1.0;netstandard2.0</TargetFrameworks>

This reduces the depedency graph of additionally installed nugets if I install this plugin in a netstandard2.0 project.