Transform the undo/redo stacks against non-user initiated changes

[thomsbg]

This allows one to mark changes that should be "undoable" by specifying source = 'user', which is useful for a collaborative editing environment in which each client maintains their own undo / redo stack (instead of sharing a global one).

[jhchen]

This is an excellent Pull Request. I've actually tried to add this myself in the past but didn't get a chance to nail down all the potential off by one errors. Just a couple of nit picks in the comments but otherwise looks great for merger.

[jhchen]

Can you rename origin to source? That's the more common name in other parts of the codebase.

[thomsbg]

Nits addressed.

[benbro]

Is there a setting to configure undo to be global or per user?
I can see the benefit of undoing only your edits but in my use case I prefer that the user will be able to undo all changes.

[thomsbg]

Fair point @benbro. I'll make it configurable.

[thomsbg]

Updated to make this new behavior configurable via a userOnly: true option

[benbro]

Thank you for the userOnly option.

[jhchen]

Since the default is userOnly: false can we change the tests back to what they were and just modify this in the new test?

[thomsbg]

Test updated.

[jhchen]

Great thanks!