[Feature] support `netstandard1.3` +

[candoumbe]

Hi !
I am a user of the Optional library and have some of librairies I wrote heavily depends on it. As it seems to no longer being maintained, I found your fork and consider moving to it.

Could there be by any chance a support for netstandard1.3 + ?

I can even contribute (if you accept contribution of course) !!