Add DesignTime code generation support for TagHelpers.

- We now generate a subset of the TagHelper runtime code during DesignTime. This enables users to see errors in the editor
- Added tests to validate design time code generation.
- Refactored runtime code generation tests (we now use a lot of their infrastructure for the design time pieces).

#208

/cc @dougbu 
`@addtaghelper` has the default behavior (add functionality to the page) which is tested in every test, no need for it to be specified directly).  `@removetaghelper` is not the default behavior and is not tested in the other scenarios.