Add a `data` field to Schema objects
Add a `data` field to Schema objects

Closes #290

salzhrani/prosemirror,salzhrani/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,tzuhsiulin/prosemirror,kiejo/prosemirror,tzuhsiulin/prosemirror,salzhrani/prosemirror,kiejo/prosemirror