Add support for disabling the autocomplete popup

[mvysny]

Thank you for developing such a useful component 👍

Sometimes I'd like to prevent the autocomplete dialog from opening, e.g. when I know upfront that the search is going to be very slow because there is insane amount of data to search from.

My first idea would be to have a setPopupEnabled() function; when called with false it would completely prevent the popup from being displayed, effectively reducing the autocomplete component to a mere text field.