Convert .net core to .net standard

[jcdekoning]

Hi Jason,

I am trying to convert some project libraries to .Net standard 2.0. The problem is that this is not possible for the libraries using FeatureToggle. While FeatureToggle supports .Net core, you cannot use these within a .Net standard project.

I created this PR whereby I converted the .Net Core FeatureToggle library to .Net standard.

Maybe you can have a look at it and if possible publish a new version?