Create a .NETStandard2.0 release

[manfred-brands]

We need to be able to release a .netstand2.0 release so that we actually release the DiagnosticSuppressors.
Current suggestion is to use 2 versions, with a different major version.

We need to update the build.cake script to run two 'pack' commands.

[manfred-brands]

@mikkelbu Have you had any more thoughts on this?

[mikkelbu]

@manfred-brands I had planned to do a release around new year with version 2.0 for the original build and 3.0 for the .netstand2.0 build, but the days have been packed with activities. I'll take a look at your PR, and then see if I can make time the coming days for a new release.

[manfred-brands]

Thanks @mikkelbu

[mikkelbu]

No problem. You did most of the work, I just had to find time to review and publish :)