Feature: simplify subapplication's definition

[why520crazy]

Now, we define a subapplication as:

defineApplication('app1', (portalApp: PlanetPortalApplication) => {
    return platformBrowserDynamic([
        {
            provide: PlanetPortalApplication,
            useValue: portalApp
        }
    ])
        .bootstrapModule(AppModule)
        .then(appModule => {
            return appModule;
        })
        .catch(error => {
            console.error(error);
            return null;
        });
});

Expect define as:

defineApplication('app1', {
   platformRef: platformBrowserDynamic(),
   AppModule: AppModule,
   // or 
   bootstrap:(options) => {
     return platformBrowserDynamic()
        .bootstrapModule(AppModule)
        .then(appModule => {
            return appModule;
        })
   }
   template: `<app1-root></app1-root>`
   Router: Router,
  // ... and somethings
});

at present, it is not appropriate to determine how to define API, look forward to your good suggestions.

The single-spa for Angular as below:
https://github.com/CanopyTax/single-spa-angular/blob/master/src/browser-lib/single-spa-angular.ts