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: main caller (`TagHelperRunner`) could use a `TagHelperOutput(string, IDictionary<string, string>, string)` overload.  this overload, like the one above, could remain but be "Internal for testing".