[Design] Add TagHelperManager for TagHelper runtime.

- The TagHelperManager is injected into the RazorPage and is used to render Tag Helpers.  Specific examples can be seen in this PR: https://github.com/aspnet/Razor/pull/135
- Tested out the stack based tracking of tag helpers and their corresponding attributes.

#130
Naming of the public methods in this file will be determined via: https://github.com/aspnet/Razor/pull/134/files#r17514939