Minor fixes to fix issues with replaying

When replaying an animation the completed state is never reached as the
property is not reset after `play()` is called subsequent times. I've
changed the `play()` call to reset this value to `false`.

I also looked into the issues reported for Promises and realized we
could do a better job when resetting it. This is where I identified the
previous problem.

I do not believe any API changes will break and I've reviewed the
examples and everything looks fine.

Closes the following issues:

- juliangarnier#373
- juliangarnier#378
- juliangarnier#392

And the following redundant PR:

- juliangarnier#408

anime.js

@@ -707,14 +707,16 @@
 
     let resolve = null;
 
-    function makePromise() {
-      return window.Promise && new Promise(_resolve => resolve = _resolve);
+    function makePromise(instance) {
+      const promise = window.Promise && new Promise(_resolve => resolve = _resolve);
+      instance.finished = promise;
+      return promise;
     }
 
-    let promise = makePromise();
-
     let instance = createNewInstance(params);
 
+    let promise = makePromise(instance);
+
     function toggleInstanceDirection() {
       instance.reversed = !instance.reversed;
     }
@@ -855,7 +857,7 @@
             setCallback('complete');
             if ('Promise' in window) {
               resolve();
-              promise = makePromise();
+              promise = makePromise(instance);
             }
           }
         }
@@ -899,6 +901,7 @@
     instance.play = function() {
       if (!instance.paused) return;
       instance.paused = false;
+      instance.completed = false;
       startTime = 0;
       lastTime = adjustTime(instance.currentTime);
       activeInstances.push(instance);
@@ -917,8 +920,6 @@
       instance.play();
     }
 
-    instance.finished = promise;
-
     instance.reset();
 
     if (instance.autoplay) instance.play();