Changes the opacity of the cell view's `$el`.

When a cell is highlighted with the opacity highlighter, its `$el` is given the `.joint-highlight-opacity` class. To customize the look of this highlighted state, you can add custom CSS rules that affect this class name.

This highlighter does not currently have any options.

Example usage:

    cellView.highlight(null/* defaults to cellView.el */, {
        highlighter: {
            name: 'opacity'
        }
    });