Configure host to utilize tag helper types.

- Set the appropriate GeneratedTagHelperContext properties to ensure code is generated correctly.

#1165
So this kind of stinks that we need to `new` this up in order to enable tag helpers.  The other alternative would be to provide a `TagHelperRuntimeEngineHost` in the `Microsoft.AspNet.Razor.Runtime` project that does this for us and then inherit from that instead of `RazorEngineHost`.  I thought it might be a little heavy handed so feel free to share your own opinions :smile: