[IMP] components: add hooks mechanism

part of #194

src/core/observer.ts

@@ -33,7 +33,7 @@ export class Observer {
     }
   }
 
-  observe(value: any, parent?: any): any {
+  observe<T>(value: T, parent?: any): T {
     if (value === null || typeof value !== "object" || value instanceof Date) {
       // fun fact: typeof null === 'object'
       return value;