WIP: Add options hash to model.save

**WIP - Don't merge or close please**

The hash is passed to `model.save({ /* .. */ });` and is sent to the server.

Questions to address:
- [ ] Should the adapter accept another argument, or pass via model as `model._options` or something.
- [ ] Should options be passed via QPs or the body, e.g. `{ meta: { /* here */ } }` or directly for delete

cc @igorT @Grapho
In regards to question 1, send to adapter and introduce a new parameter there, or attach to the record?