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 
general question: why bother turning off the generation of this code?  IOW how does this mess up design-time?