paging: Add support for setting listitem-HTML in the options

Usage (works with bootstrap pagination):

```
var filterPagingOptions = {
    outerWindow: 1,
    innerWindow: 1,
    itemHtml: '<li><span class="page"></span></li>'
};
var filterOptions = {
    valueNames: ['name', 'date'],
    page: 50,
    plugins: [
        [ 'paging', filterPagingOptions ],
    ]
};
```