Remove `value` from `operation.fromJSON` (#1451)
Remove `value` from `operation.fromJSON` (#1451)

* Remove `value` from `operation.fromJSON`

Value is only used for local undo / redo, so it shouldn't be
serialized along with the rest of the data.

* Continue to parse a specified `value` into an Operation

ashutoshrishi/slate,ianstormtaylor/slate,ianstormtaylor/slate,isubastiCadmus/slate