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
nit: `/// <c>string.Empty</c> if <see cref="TagName"/> is <c>string.Empty</c/>.  Otherwise, the <see cref="string"/> representation ...`