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/test/public_api_spec.ts

@@ -27,6 +27,7 @@ import {SymbolsDiff} from './symbol_inspector/symbol_differ';
 
 var NG_ALL = [
   'APP_COMPONENT',
+  'APP_INITIALIZER',
   'APP_ID',
   'AbstractProviderError',
   'AbstractProviderError.addKey()',
@@ -1396,6 +1397,7 @@ var NG_ALL = [
   'resolveForwardRef():js',
   'wtfCreateScope():js',
   'PLATFORM_COMMON_PROVIDERS',
+  'PLATFORM_INITIALIZER',
   'wtfCreateScope:dart',
   'wtfEndTimeRange():js',
   'wtfEndTimeRange:dart',