lib/input-generators/sitemap-index.js
function prepareWrite (outputPath, callback) {
  var path = pathLib.parse(outputPath);
  var dir = pathLib.join(path.root, path.dir);
  mkdirp(dir, callback);
}
function prepareWrite (outputPath, callback) {
  var path = pathLib.parse(outputPath);
  var dir = pathLib.join(path.root, path.dir);
  mkdirp(dir, callback);
}
Make sure path exists so a sitemap write can succeed.

@param {String} outputPath - File path and name.
@param {Funcion} callback - Completion callback.