Added a minimum query length constrain to autocomplete

If user types a single chararchter and pauses, the auto-completeion will kick off and fetch a very large number of suggestions, which ofter causes the UI to hang. Therefore I added a option to constrain the minimal length for query strings.
It's not super important but as you can see in the rest of the file the styling convention is "if[SPACE]([STUFF])[SPACE]{" so it should ideally be:
```
if (textbox.val().trim().length < options.minLength) {
```
It might not seem important but code that is a mix of formatting looks broken and new developers looking at it might think this isn't supported or that correctness isn't important.