[Design] Add Assets: TagHelperRazorExpression, TagHelper, TagHelperRenderingContext,  TagHelperChunk and related mid-pipeline generation code.

- Added TagHelperRazorExpression
- Added base TagHelper class
- Added TagHelper specific rendering context.
- Added pre-code generation pieces such as TagHelperChunk and TagHelperCodeGenerator.
- These are not immediately used but will be used for #72 and Mvc specific implementations.  Felt that I should break these standalone pieces into their own PR since they're pretty much just "assets".

#68
Completely empty abstract classes/interfaces are always a bit smelly. What is actually common among TagHelpers? Who inherits from this class?