Update MMDrawerController.m

Here is another way to support editing/dragging cells in the center view controller. 
Make open pan gesture enable only at both sides, even at the edge of the container view.
MMDrawerSidePanGestureFlag & MMDrawerPanGestureSideContainerWidth are configurable to fit different situation.
No need to add those lines as Git Blame will properly identify the author of any given line.