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
(nice infinite recursion but) please write a `<summary/>` that says something instead e.g.
`/// The <see cref="ITagHelper">s' output.` (note placement of apostrophe) or
`/// Output of the <see cref="TagHelperExecutionContext.TagHelpers">` (need to distinguish property from type)