Save and restore writable attributes as well

[tannewt]

From @Apollon77 on #7:

How did you decided if an attribute is non volatile? If you just use the spec flagging you might not have all needed ones.

Matter.js uses the best practice: When an attribute is writable OR non-volatiove then we persist it. For some writable attributes the Non-volatile flagging is missing but in fact if these are config then it only makes sense to also persist them- Spec is a bit vague here because it states that non-volatile should be persisted or "when needed" :-)

So maybe you should think to also include writable ones