Suggestion: Allow message lines to be more obviously in front of Actor lines

[tstarbow-sfdc]

One of the things I like about sequencediagrams.org's renders is the way that the message text and lines are obviously in front of the Actor lines. Visually, this just means adding some background color around the Message Text and Message Lines.
I was able to do this in css for the Message Text, by setting the stroke to the background color and using fill for the drawing color. But this doesn't work for the Message Lines because they don't take fill.
One approach would be to draw the line twice, first with a MessageLineBack class.
Another approach would be to make the MessageLines rect's instead of lines.
Either of these should leave rendering as is by default, but allow css to define the visual separation.

If there is any appetite for this idea, I would be up for coding a proposed pull request.