Add Core TagHelper assets

- 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
It's not new here, but in general should avoid using just `T` for generic arguments unless it truly could be any type. For example, a better name for this might be `TChunk` or similar. Might want to consider a cleanup PR after this (don't want to jumble it into this PR).