Add a meta field for nodes to freely store data

[pierpo]

Is your feature request related to a problem? Please describe.

We're integrating LRUD in a react-native app, and we frequently need to access the data linked to our node. Most notably the ref of our element, and some other stuff.

Concerning the ref: the example on the README says that we simply could get our element in the DOM by id document.getElementById(id) to get its ref. But as we're using React Native, this is not possible!
Ideally, we'd love to store the ref in a meta field of the node.

Describe the solution you'd like

It would be lovely to simply be able to store meta data in the node.

lrud.registerNode(id, {
  meta: {
    anything: 'I want',
  }
})

Describe alternatives you've considered

We had no other idea 🤷

Additional context

None yet.

(also: THANK YOU for this amazing lib. You've done an amazing job, and this is a lifesaver ❤️ )

[pierpo]

Btw, I can submit a PR if you think the feature request is relevant 😊