function push(dest, name, elem) {
  if (dest[name] === undefined) dest[name] = [elem];
  else dest[name].push(elem);
}
Adds an offer to the map of extension offers or a parameter to the map of
parameters.

@param {Object} dest The map of extension offers or parameters
@param {String} name The extension or parameter name
@param {(Object|Boolean|String)} elem The extension parameters or the
    parameter value
@private