Add support for ViewStart

* Modify MvcRazorHost to generate injected properties from ViewStart into
  views
* Modify RazorViews to execute ViewStart
Sort of - https://github.com/ASP-NET-MVC/aspnetwebstack/blob/master/src/System.Web.WebPages/StartPage.cs#L101-L141. They had the advantage of using the VPP to provide an abstraction over the app root. 

I tried using the IFileSystem to start with, but that doesn't have a way to tell me that I have the site root. It does return false for any paths above the root path, but that's not sufficient information to determine if we've hit the app root.