Adds `autofocusInputOnOpen` option.

For issue #1541:

* `autofocusInputOnOpen` is a boolean option that specifies if the input
  search field should grab focus when the dropdown is opened.  It
  defaults to true.  This can be used for mobile applications where the
  virtual keyboard should not immediately be displayed when the dropdown
  is opened.
* The input search field will not grab focus if
  `minimumResultsForSearch` is negative.