automatically focusing input field

Hello,
Instead of this fix we can add `window.NLField = NLField;` as [here](https://github.com/creativewebsrl/NaturalLanguageForm/blob/master/js/nlform.js#L217) and override `NLField::_open` method :

    var _open = NLField.prototype._open;
    NLField.prototype._open = function () {
        _open.apply(this, arguments);

        var self = this;
        if (this.type === 'input' ) {
            setTimeout(function() {
                self.getinput.focus(); // or self.getinput.select();
            }, 100);
        }
    }