Add a promise-driven initAsync method

Allows the loading of resources (such as web assembly modules)
before initializing Crafty.

src/core/core.js

@@ -1384,6 +1384,7 @@ Crafty._addCallbackMethods(Crafty);
 Crafty.extend({
     // Define Crafty's id
     0: "global",
+
     /**@
      * #Crafty.init
      * @category Core
@@ -1426,6 +1427,64 @@ Crafty.extend({
         return this;
     },
 
+    /**@
+     * #Crafty.initAsync
+     * @category Core
+     * @kind Method
+     *
+     * @trigger Load - Just after the viewport is initialised. Before the UpdateFrame loops is started
+     * @sign public this Crafty.initAsync([Array loaders], [Number width, Number height, String stage_elem])
+     * @param Array loaders An array that can contain a mix of Promises, or functions that Promises
+     * @param Number width - Width of the stage
+     * @param Number height - Height of the stage
+     * @param String or HTMLElement stage_elem - the element to use for the stage
+     *
+     * A version of Crafty.init that uses Promises to load any needed resources before initialization.
+     *
+     * The first argument is an array of either Promises, or functions that return promises.
+     * In the second case, a Promise will be generated by invoking the function with the current Crafty instance as an argument.
+     * (The array can be a mixture of the two types.)
+     *
+     * Once all promises are ready, and the document itself has loaded, Crafty.init will be invoked with the `w`, `h`, and `stage_elem` arguments.
+     *
+     * The method returns a Promise whose fufillment is an array of each passed promises fufillment,
+     * and whose rejection is the rejection of the first promise to fail.  (Essentially the same as Promise.all)
+     *
+     * @see Crafty.init
+     */
+    initAsync: function(loaders, w, h, stage_elem) {
+        // first argument is optional
+        if (typeof loaders !== "object") {
+            stage_elem = h;
+            h = w;
+            w = loaders;
+            loaders = [];
+        }
+
+        var promises = [];
+        for (var i in loaders) {
+            if (typeof loaders[i] === "function") {
+                promises.push(loaders[i](Crafty));
+            } else {
+                promises.push(loaders[i]);
+            }
+        }
+
+        // wait for document loading if necessary
+        if (document.readyState !== "complete") {
+            var window_load = new Promise(function(resolve, reject) {
+                window.onload = resolve;
+                promises.push(window_load);
+            });
+        }
+
+        return Promise.all(promises).then(function(values) {
+            Crafty.init(w, h, stage_elem);
+            // Return results only for promises passed in with the loaders array
+            return values.slice(0, loaders.length);
+        });
+    },
+
     // There are some events that need to be bound to Crafty when it's started/restarted, so store them here
     // Switching Crafty's internals to use the new system idiom should allow removing this hack
     _bindOnInit: [],