Changes the opacity of an arbitrary cell view's SVG node.

When a cell is highlighted with the opacity highlighter, the node determined by the selector 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:

    joint.highlighters.opacity.add(cellView, 'body', 'my-highlighter-id');