add loading state class to input

[ofrias]

It would be useful to add a "loading" state class to the autocomplete input that could be used to add for example a background spinner to indicate that the autocomplete is loading data. This can be done easily with this code in the startFetch function:

            input.classList.add('ac-loading');  // new code
            settings.fetch(inputText, function (elements) {
                if (fetchCounter === savedFetchCounter && elements) {
                    input.classList.remove('ac-loading');  // new code

Then it can be used with CSS code like this:

.ac-loading {
  background-image: url('path to spinner image');
  background-size: 25px 25px;
  background-position: right 10px center;
  background-repeat: no-repeat;
}