prevent losing the properties in a dot file that is not generated by Kite

[erkal]

idea: since there will undoubtedly be graphviz properties that aren't first-class properties in Kite, we could hold these extraneous properties in a Dict String String. that way, we should be able to import a .dot file and then export an equivalent .dot, without losing those properties.

i haven't looked at how feasible this is at an implementation level. maybe it complicates things more than it's worth.

Originally posted by @brandly in https://github.com/erkal/kite/issues/4#issuecomment-480003242

[erkal]

https://github.com/erkal/kite/issues/4#issuecomment-480167724