code: function init(options = {}, handlebars) {
    const opts = deepExtend({}, defaults, options);
    normalizePaths(opts);
    opts.handlebars = handlebars || Handlebars.create();
    return Promise.resolve(opts);
  }
docs: Merge defaults into passed options.
  @param {Object} options
  @param {Object} handlebars   Handlebars instance—it can be passed explicitly,
  primarily for testing purposes.
  @return {Promise} resolving to merged options