feat(bootstrap): add platform and app initializers

Often some init logic needs to run when a platform or an application is boostrapped.
For example, boostraping a platform requires initializing the dom adapter.
Now, it can be done as follows:

new Provider(PLATFORM_INITIALIZER, {useValue: initDomAdapter, multi: true}),

All platform initializers will be run after the platform injector has been created.

Similarly, all application initializers will be run after the app injector has been
created.

Closes #5355

modules/angular2/platform/browser_static.ts

@@ -12,8 +12,7 @@ import {Type, isPresent, CONST_EXPR} from 'angular2/src/facade/lang';
 import {Promise} from 'angular2/src/facade/promise';
 import {
   BROWSER_PROVIDERS,
-  BROWSER_APP_COMMON_PROVIDERS,
-  initDomAdapter
+  BROWSER_APP_COMMON_PROVIDERS
 } from 'angular2/src/platform/browser_common';
 import {ComponentRef, platform, reflector} from 'angular2/core';
 
@@ -31,7 +30,6 @@ export const BROWSER_APP_PROVIDERS: Array<any /*Type | Provider | any[]*/> =
 export function bootstrapStatic(appComponentType: Type,
                                 customProviders?: Array<any /*Type | Provider | any[]*/>,
                                 initReflector?: Function): Promise<ComponentRef> {
-  initDomAdapter();
   if (isPresent(initReflector)) {
     initReflector();
   }