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
teensy nit: no need for "TagHelpersExecutionContext" in these test names.  here you're testing "AddHtmlAttribute" so "AddHtmlAttribute_MaintainsHtmlAttributes" works.