feat(prototype): allow prototype value to be a function

By supplying a function as the value of `prototype` you can now alter the
prototype chain.

BREAKING CHANGE: We've retired `proto` as it can be better implemented
with a callback.

prototype.js

@@ -9,18 +9,18 @@
     }));
   }
 })([], this, function () {
-  function proto() {};
+  var toString = Object.prototype.toString;
 
   return function (result, data) {
-    if (data.key === "proto") {
-      proto.prototype = data.value;
-      result.prototype = new proto();
-    }
-    else if (data.key === "prototype") {
-      result.prototype = data.value;
+    var value = data.value;
+
+    if (data.key === "prototype") {
+      result.prototype = toString.call(value) === "[object Function]"
+        ? value.call(this, result)
+        : value;
     }
     else {
-      result.prototype[data.key] = data.value;
+      result.prototype[data.key] = value;
     }
   }
 });

tests/create.js

@@ -133,23 +133,59 @@
 
   QUnit.module("mu-create/create#property");
 
-  QUnit.test("proto instance", function (assert) {
+  QUnit.test("prototype instance", function (assert) {
     assert.expect(1);
 
     var p = {};
     var C = create(proto)({
-      "proto": p
+      "prototype": p
     });
 
-    assert.strictEqual(C.prototype.__proto__, p, "instance is equal");
+    assert.strictEqual(C.prototype, p, "instance is equal");
   });
 
-  QUnit.test("prototype instance", function (assert) {
+  QUnit.test("prototype function", function (assert) {
+    assert.expect(1);
+
+    var C = create(proto)({
+      "prototype": function() {
+        assert.ok(true, "callback called");
+      }
+    });
+  });
+
+  QUnit.test("prototype function arguments", function (assert) {
+    assert.expect(1);
+
+    var p;
+    var C = create(proto)({
+      "prototype": function(c) {
+        return p = c.prototype;
+      }
+    });
+
+    assert.strictEqual(p, C.prototype, "p matches C.prototype");
+  });
+
+  QUnit.test("prototype function scope", function (assert) {
+    assert.expect(1);
+
+    var s = {};
+    var C = create(proto).call(s, {
+      "prototype": function() {
+        assert.strictEqual(this, s, "scope matches");
+      }
+    });
+  });
+
+  QUnit.test("prototype function return", function (assert) {
     assert.expect(1);
 
     var p = {};
     var C = create(proto)({
-      "prototype": p
+      "prototype": function() {
+        return p;
+      }
     });
 
     assert.strictEqual(C.prototype, p, "instance is equal");