pillbox-attributes: Add attributes object to addItems method

[interactivellama]

This Pull request replaces #692 for issue #679.

Allows:

$('#myPillbox1').pillbox('addItems',
    {
        text: 'item 1',
        value: 1,
        attr: {
                'cssClass': 'example-pill-class',
                'style': 'background-color: ' + backgroundColor + ';',
                'data-example-attribute': 'true'
            }
    });

amd $('#myPillbox1').pillbox('items') returns:

{
    text: 'item 1',
    value: 1,
    exampleAttribute: true
};

Text and data attributes are the only attributes returned.

[kevinparkerson]

looks fine to me