Allow adding markup to elements

[chfin]

In the annotation tool, allow the user to add some form of markup to notes, slices, and edges.

The simplest idea would be to add custom CSS classes that can be styled independently. This would make it much easier to derive a figure from the annotation by adding CSS to the exported SVG. A certain set of classes (e.g. colors, highlighted, "muted") could be predefined in both the annotation tool and the viewer, without the need to define custom styles to see the classes.

This extra information would be stored as a dictionary in a separate top-level key of the JSON file, so that it's easy to ignore. This section could also be used to specify general settings, e.g. the default settings for the viewer.