Add code generation tests for tag helpers.

#72 

Implementation PR: https://github.com/aspnet/Razor/pull/134
Gonna wait on @DamianEdwards feedback.  We wanted this to be purely a runtime rendering decision.  Also note: Another somewhat unique behavior of the `TagHelperOutput` object is setting the `TagName` to be empty/null which then results in no start or end tag being rendered, allowing a user to do whatever they want in the `Content`, this would enable your `<input type="checked" .../>` requirements.