Support string numbers for propTypes

PropTypes only took a number for `staggerDurationBy`, but it feels
weird to specify a number for a prop. Better to accept numbers or
strings, and do the conversion within (which is done by multiplying
the number).

package.json

@@ -1,6 +1,6 @@
 {
   "name": "react-flip-move",
-  "version": "0.1.3",
+  "version": "0.1.4",
   "description": "Effortless animation between DOM changes (eg. list reordering) using the FLIP technique.",
   "main": "lib/index.js",
   "files": [

src/FlipMove.js

@@ -118,7 +118,10 @@ class FlipMove extends Component {
     direction:  PropTypes.string,
     fill:       PropTypes.string,
     onComplete: PropTypes.func,
-    staggerDurationBy: PropTypes.number
+    staggerDurationBy: PropTypes.oneOfType([
+      PropTypes.string,
+      PropTypes.number
+    ])
   };
 
   static defaultProps = {