Add TagHelperRunner for TagHelper runtime.

- The TagHelperRunner is responsible for calling into the process methods of the TagHelpers.
- This involved adding the following core classes: TagHelper, TagHelperExecutionContext, TagHelperOutput, TagHelperContext.  All of which aid in running TagHelpers.
- Tested TagHelperRunner, TagHelperOutput and TagHelperExecutionContext.

#154
search everywhere for the words "define" and "represent"; find a better way to phrase where they appear.  here it's `/// A task that on completion updates the <paramref name="output"/>.`