Allow pre-process function to cancel caching

And the corresponding pre-process function that an i18n site would then use would be:

/**
 * Boost pre-process function.
 *
 * We use this to avoid caching non-i18n prefixed URLs.
 *
 * @arg string $data
 *  What Boost is about to cache to disk.
 * @arg string $path
 *  What URL are we at.
 * @return string
 *  Either $data, or nothing.
 */
function project350_boost_pre_process($data, $path) {
  if (module_exists('i18n') && i18n_get_lang_prefix($path)) {
    return $data;
  }
}