Save plugin

[zbeyens]

• a lot of people are asking how to save only diffs
• there is no way for plugins to filter onChange value
• too much code end up in onChange
• a plugin is adding UI props into slate ast that should be removed before saving (not on each onChange call)
• there is no method to manually save the editor state

I think onChange scope is too broad but we should keep its original slate behavior.

Thinking to introduce withSave with an overridable editor.save that can be run anywhere: in onChange or manually

[zbeyens]

Duplicate of #88