code: function parseLayout(str, filename, cb) {
      var layoutFile = self.declaredLayoutFile(str, filename);
      if (layoutFile) {
        self.cacheLayout(layoutFile, options.cache, cb);
      } else {
        cb(null, null);
      }
    }
docs: Allow a layout to be declared as a handlebars comment to remain spec
  compatible with handlebars.
  
  Valid directives
  
  {{!< foo}}                      # foo.hbs in same directory as template
  {{!< ../layouts/default}}       # default.hbs in parent layout directory
  {{!< ../layouts/default.html}}  # default.html in parent layout directory