add shouldStretchDrawer property to drawerStyle (#1817)

docs/top-level-api.md

       contentOverlayColor: 'rgba(0,0,0,0.25)', // optional, add this if you want a overlay color when drawer is open
       leftDrawerWidth: 50, // optional, add this if you want a define left drawer width (50=percent)
       rightDrawerWidth: 50 // optional, add this if you want a define right drawer width (50=percent)
+      shouldStretchDrawer: true // optional, iOS only with 'MMDrawer' type, whether or not the panning gesture will “hard-stop” at the maximum width for a given drawer side, default : true
     },
     type: 'MMDrawer', // optional, iOS only, types: 'TheSideBar', 'MMDrawer' default: 'MMDrawer'
     animationType: 'door', //optional, iOS only, for MMDrawer: 'door', 'parallax', 'slide', 'slide-and-scale'

ios/RCCDrawerController/RCCDrawerController.m

         UIColor *color = contentOverlayColor != (id)[NSNull null] ? [RCTConvert UIColor:contentOverlayColor] : nil;
         [self setCenterOverlayColor:color];
     }
+    
+    if (self.drawerStyle[@"shouldStretchDrawer"]) {
+        self.shouldStretchDrawer = ([self.drawerStyle[@"shouldStretchDrawer"] boolValue]) ? YES : NO;
+    }
 }