Correctly handle extension tags in template output.

* Extension tags in template output were being ignored.
* Updated tokenizer to record source of extension content on the
  start tag and used it in the extension collector to create an
  extension token wrapper with the content.
* Unrelated fix: Deleted unused productions in tokenizer.
* No change in parser tests, but {{echo|<math>foo</math>}} is now
  parsed and rt-ed properly.

Change-Id: Id5f69e71f57255a86bc3c069a5d929b06c665d31
Merge "Correctly handle extension tags in template output."