    util.setAttributesBySelector(el, attrs)

Set attributes on the DOM element (SVG or HTML) `el` and its descendants based on the selector in the `attrs` object. The `attrs` object is of the form: `{ [selector]: [attributes }`. For example:

    var myEl = document.querySelector('.mydiv');
    joint.util.setAttributesBySelector(myEl, {
        '.myDiv': { 'data-foo': 'bar' },    // Note the reference to the myEl element itself.
        'input': { 'value': 'my value' }
    });

The code above sets `'data-foo'` attribute of the `myEl` element to the value `'bar'` and `'value'` attribute of all the descendant inputs to the value `'my value'`. This is a convenient way of setting attributes on elements (including themselves) and their descendants. Note that `'class'` attribute is treated as a special case and its value does not override the previous value of the `'class'` attribute. Instead, it adds this new class to the list of classes for the element referenced in the selector.