[IMP] config: add enableTransitions flag to config

It is useful to be able to disable the t-transition directive for
testing purposes.  This commit introduces a global config flag to do
just that.

closes #768

src/config.ts

@@ -8,6 +8,7 @@ import { QWeb } from "./qweb/index";
 
 interface Config {
   mode: string;
+  enableTransitions: boolean;
 }
 
 export const config = {} as Config;
@@ -28,3 +29,12 @@ Object.defineProperty(config, "mode", {
     }
   },
 });
+
+Object.defineProperty(config, "enableTransitions", {
+  get() {
+    return QWeb.enableTransitions;
+  },
+  set(value: boolean) {
+    QWeb.enableTransitions = value;
+  },
+});