NetStandard2.0 optimizations

[sebastienros]

MimeKit target netstandard 2.0 but the csproj should not reference the assemblies it does right now with this target. Only Bouncy should still be necessary but it looks like this other ones are not. This would make the dependency graph smaller when using it from an app targeting netcoreapp2.0.

I can work on a PR if you agree.

[jstedfast]

Yea, I've known about this for a bit but haven't gotten around to fixing it. Thanks!

[jstedfast]

Do you have an idea of when you might have something ready? Just trying to plan out when to make the next release.

[sebastienros]

Are the pull requests going through CI for each target? If so they I don't need to run the tests myself

[jstedfast]

No, they only test 4.5

[sebastienros]

At least it will build the project, so it would just fail if there were issues. Will try today.