Custom autosave handler

### I'm submitting a...
- [ ] Bug report
- [x] Feature request

The current autosave feature periodically stores contents to the local storage. It would be great if a function for autosave can be passed to option so that the autosave handler may be overridden for feature like server-side autosave,

Start by locating the existing autosave option and the code that periodically writes editor contents to local storage. Add support for a caller-provided autosave function while preserving the current behavior when none is supplied; done means a custom handler can support server-side autosaving.

Written by the indexing model from the issue text.