Added node handler for partials and simple partial helper.

  - Partial node handler pushed partial token to our html parser.
  - Partial token is aliased to partial helper in hydration opcodes.
  - Added helper object to env in rendered templates
  - Added helper object to env in html_compiler tests.
  - Added registerHelper function to html_compiler tests.
  - Fleshed out tests for both htmlbars style partials and helper style partials