Tag Helpers: Enable TagHelpers in MvcRazorHost

[NTaylorMullen]

Razor exposes a TagHelperDescriptorResolver in aspnet/Razor#148 which needs to be created from the runtime host in order to enable TagHelpers.

This item involves:

• newing up a TagHelperDescriptorResolver for the MvcRazorHost to use.
• Passing in the appropriate string type names down to Razor.