Add a way to prevent hide action.

In our app development, we found it necessary to have a way to block the hiding of the dropdown programmatically for particular cases. This is what we are using in our code, and I thought it might be useful.

And then we use it is, for example in some function:

```javascript
function doSomeActionWhileDropdownIsOpen() {
    var dropdown = $('#search-dropdown');
    dropdown.addClass('prevent-dropdown');
    // do things (e.g. allow user to click places)
    dropdown.removeClass('prevent-dropdown')
}
```