Add a .NET Standard 2.0 target

[f2calv]

First a great library thanks :)
However it appears to be a unique Serilog library in that its the only one that isn't backward compatible with .NET Standard 2.0;

[nblumhardt]

Wowsers, that's quite a few Serilog package dependencies you have there! 😅

I'm open to adding netstandard2.0 support if we can do it without degrading the quality of the codebase; it's been a while since I looked into it, so might need some experimentation - would be great to know how much needs to change, if you have the time to try it out!

Otherwise, I'll loop back around and take a look as soon as I can. Cheers!

[f2calv]

Already done ;) (I hope)