Initial Roslyn support.

I will merge this after https://github.com/Antaris/RazorEngine/pull/204 (that's why the #204 commits are included here).
This adds support for roslyn via an additional nuget package (RazorEngine.Roslyn). I had to change some minor internals to make it work flawlessly.
The documentation for this is: https://matthid.github.io/RazorEngine/Roslyn.html
Develop Intro was also updated: https://matthid.github.io/RazorEngine/DevelopReadme.html
(The docs will be updated to https://antaris.github.io/RazorEngine/ once we release the nuget packages, as the docs are wrong for the current nuget package!)

This adds some nuget packages:
- `RazorEngine.Roslyn.3.5.0-beta1`: Roslyn Extensions for the `RazorEngine.3.5.0-beta1` package
- `RazorEngine.Roslyn.4.0.0-beta1`: Roslyn Extensions for the `RazorEngine.4.0.0-beta1` package