code: function getOption(opts, key, def) {
    var value = opts[key];
    return value === undefined ? def : value;
  }
docs: Get option or default if undefined.
  
  @param opts {Object} Options.
  @param key {String} Name of the option.
  @param def {...} Default value.
  
  This is useful mostly for true-ish defaults and false-ish values (where the
  usual `||` idiom breaks down).