substitute(content, url) {
    var relUrls;

    if (url) {
      relUrls = this.relativeTo(url);
    } else {
      relUrls = this.urls;
    }

    return Object(replacements["e" /* substitute */])(content, relUrls, this.replacementUrls);
  }
Substitute urls in content, with replacements,
relative to a url if provided
@param  {string} content
@param  {string} [url]   url to resolve to
@return {string}         content with urls substituted