Make rootUrl customization easier

[vadjs]

Current implementation of rootUrl customization is not very user friendly.
With this changes you can use custom rootUrl much easier. Something like this:

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    ApiModule.forRoot({rootUrl: 'https://some-root-url.com'}),
  ],
  bootstrap: [
    AppComponent
  ]
})
export class AppModule { }

[vadjs]

@luisfpg I hope you not abandoned this repo.

[luisfpg]

@vadjs No, I haven't abandoned. I have plenty of other tasks, so I maintain this project on free time.
I've already started https://github.com/cyclosproject/ng-openapi-gen, but ng-swagger-gen will still be maintained for bug fixes and improvements such as this one you sent.