Allow to set widget tabs per API

[cguedes]

The current implementation only allows setting custom tabs globally. It would be nice to customize those per API.

APIExplorer
  .addAPI('petstore', 'swagger2', 'https://api.swaggerhub.com/apis/anil614sagar/petStore/1.0.0', c => {
    c.addWidgetTab('Only For Petstore', APIExplorer.PetstoreTabWidget)   // <-- this is the new configuration
  })
  .addWidgetTab('Global', APIExplorer.GlobalTabWidget)
  .start()