Create and contribute a color class for .NET

[craigfowler]

I want to create and contribute (to Selenium) something like the following, but for .NET.

https://github.com/SeleniumHQ/selenium/blob/trunk/java%2Fsrc%2Forg%2Fopenqa%2Fselenium%2Fsupport%2FColor.java

I'll need to initially create it in the Screenplay repo, but no reason not to share it back to Selenium with a PR.

In turn, this will allow me to fix cross-browser tests which fail because they report colour differently.

[craigfowler]

I'd like to make this new object a struct, which is equatable and may be converted to/from the various representations which browsers use.