improve average operator

src/operators.js

@@ -21,12 +21,23 @@ of({a: 1, b:2, e: 1}, {a: 0, c: 1, b:3}, {a: 0, d: 1, b:1})
   .subscribe((v) => console.log(v));
 */
 
-function $average() {
+function $average(cb) {
     return pipe(
-    reduce((accum, curr) => ({
-      sum: accum.sum + curr,
+      reduce((accum, curr) => {
+        let currentValue;
+        if (typeof cb === 'string' || cb instanceof String){
+          currentValue = curr[cb];
+        } else if(typeof cb === "function") {
+          currentValue = cb(curr);
+        } else {
+          currentValue = curr;
+        }
+  
+        return {
+          sum: accum.sum + currentValue,
           count: accum.count + 1
-    }), { sum: 0, count: 0 }),
+        }
+      }, { sum: 0, count: 0 }),
       map(o => o.sum / o.count)
     );
   }