Get SVG strings back for "other" SVG elements

[dan-fritchman]

We essentially parse everything in an SVG schematic into two camps:

1. Schematic content
2. Anything else

Elements in camp (2) should be retained and saved back to file, and someday be editable in our editor.
Thus far we haven't really done any of that, except parse them, look at them, and drop them.
We just have their parser objects (for now), and haven't sorted out turning them back into SVG strings.

The (incredibly helpful!) parser we've used https://github.com/Rich-Harris/svg-parser has a related issue: Rich-Harris/svg-parser#14. Using that should be our answer.