how to handle arbitrary entities and attributes that don't fit in the spec?

[pketh]

Sort of connected to #55 , but if I have a document with other types of entities (like shapes I'm not sure whether adding those things as siblings to nodes and edges

{ 
  nodes: [],
  shapes: [{id, other attributes}]

would make a document non-compliant with the canvas spec , or whether canvas supporting apps would just ignore unsupported entities?

Relatedly, how should I handle custom attributes on nodes? If I make a node with a custom type like

{id: '123', type: 'list}

how would that node be handled in other canvas supporting apps? (In this case it would be fine if non-supported bits were ignored)