Fix 'Select All' when there are dividers

There's a handy (although undocumented) feature in bootstrap-multiselect where you can create horizontal dividers if you have an `<option value="" data-role="divider"></option>`.

This, however was breaking the 'Select All' functionality since the divider option could never be "checked" in the UI.

You might also have the same issue with 'disabled' options - though I haven't confirmed that.