Add callback when editing on cell is cancelled

In version 1.2.2 of react-bootstrap-table2-editor, it is possible to be notified when a cell enters the editing state using onStartEdit. However, if blurToSave is set to false, there is no way to determine that editing was cancelled when the user clicks away from the editing cell and the cell transitions back to a non-editing state. It would be very useful to have a onEditCancel (or some other appropriate name) callback method that could be set for this case. When blurToSave is true, all is fine as any change in focus, leads to the afterCellSave method to be called.