getSelectedValue(customSelect) {
    const hiddenSelect = this.UI.getHiddenSelect(customSelect);
    if (this.isMultiple(customSelect)) {
      let selectedOptions = [];
      [].forEach.call(hiddenSelect.options, option => {
        if (option.selected) {
          selectedOptions.push(option.value);
        }
      });
      return selectedOptions;
    } else {
      return hiddenSelect.options[hiddenSelect.selectedIndex].value;
    }
  }
Get selected value
If select is multiple then returns array

@param customSelect
@returns {String|Array}