Add custom IDs / class properties to paths

[ttseng]

I'm interested in being able to add custom ID / class properties to SVG elements. Is this something you could potentially support in the future?

The reason I want to do this is related to the goal in this feature request to create SVG groups: #243

Ultimately, I'd like to be able to write a script (outside of p5.js) to be able to group all items with the same class.

Example: I'm using the AxiDraw and want all items with the class blue to be in one group, and all items with the class green to be in another group. This lets me then handle plotting of individual groups by color easily.

Hopefully that request makes sense!