add strong key pair for NET Standard

[jetersen]

Hi @SimonCropp and @jbevain I followed up on the suggestion in #548

Not sure if my 54a8987 is helpful, I can easily remove it if you feel like it's a bad addition.

For our use case it meant we could remove using System.Reflection and avoid the same

#if (NETSTANDARD)
using StrongNameKeyPair=Mono.Cecil.StrongNameKeyPair;
#else
using StrongNameKeyPair=System.Reflection.StrongNameKeyPair;
#endif

In all our files that used StrongNameKeyPair.

[jbevain]

Hey!

Thanks for your contribution! Unfortunately I don't think I'll take this one, I'm not sure I want to add that to the public API and support it and have people take a dependency on Cecil for that feature.

It should be very easy to take the code needed out and extract that into a separate project.

Thanks again!