Updated MultiPolygon 'polygons' prop to allow polygons with holes (#218)

Like 'Polygon' already allows passing array of arrays to make holes, also
MultiPolygon should allow it. Even though not read in Leaflet
MultiPolygon documentation, it supports it because Polygon support it.

src/MultiPolygon.js

@@ -8,7 +8,12 @@ import Path from './Path'
 
 export default class MultiPolygon extends Path {
   static propTypes = {
-    polygons: PropTypes.arrayOf(latlngListType).isRequired,
+    polygons: PropTypes.arrayOf(
+      PropTypes.oneOfType([
+        latlngListType,
+        PropTypes.arrayOf(latlngListType),
+      ])
+    ).isRequired,
   };
 
   componentWillMount () {