make canvas argument optional on instantiating engine

test/src/engine_test.dart

@@ -0,0 +1,12 @@
+@TestOn('browser')
+
+import 'dart:html';
+
+import 'package:rules_of_living/src/Engine.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test("Engine can be instantiated without canvas", () {
+    expect(Engine(), isNot(throwsA(anything)));
+  });
+}