code: function include(partial, cb) {
      cb((params.partials && params.partials[partial]) ?
        params.partials[partial] :
        '[error] partial ' + partial + ' does not exist');
    }
docs: Include a partial template in the current page.
  An error message will be included when partial does not exist.
  
  @param {String} partial: partial template file to include
  @param {Function} cb: jazz cb(data) callback