Add Diacritic support for filtering text

[mikehins]

Hi, thanks for this awesome plugin !

I needed to filter options regardless of accents. So I added an option enableReplaceDiacritics.

$(document).ready(function () {
   $('#example-filter-enableReplaceDiacritics').multiselect({
        enableFiltering: true,
        enableReplaceDiacritics : true,
        enableCaseInsensitiveFiltering: true
    });
});
<select id="example-filter-enableReplaceDiacritics" multiple="multiple">
    <option value="a">Bière</option>
    <option value="b">birrë</option>
    <option value="c">Øl</option>
    <option value="d">pivə</option>
    <option value="e">Sör</option>
</select>```

[davidstutz]

Thanks this might be helpful!

[davidstutz]

Will manually use a mix of this PR and #725.