Move token methods to tokenizer

Prompted by suggestion in https://github.com/tildeio/htmlbars/pull/198.   This just moves the methods and temporary state (`_currentAttribute`) to the tokenizer, so that tokens themselves contain just data.  Still using constructors for them since the end-result is the same, plus the unit tests use the constructors for building expected values.

Reduces duplication a bit as some of the methods were identical or nearly so across several token types.  EDIT: And I should say that this will require changes to htmlbars as well (some trivial, but the ones concerning `tokens.js` less so, since that is overriding and adding methods).  So not sure it entirely justifies the trouble.