Loading spinner should show when using the Spec parameter

[mckennajones]

Q	A
Bug or feature request?	Feature
Which Swagger-UI version?	03.14.0
How did you install Swagger-UI?	NPM
Which browser & version?	Firefox 61
Which operating system?	Win 10

Configuration (browser query string, constructor, config.yaml)

const ui = SwaggerUI({
      spec: JSON.parse(json),
      validatorUrl: '',
      domNode: this.el.nativeElement.querySelector('.swagger-container'),
      docExpansion: 'none',
      presets: [
        SwaggerUI.presets.apis,
      ]
    });

Expected Behavior

If you supply a large JSON spec to the spec parameter, which causes Swagger-UI to take a few seconds to render, it would be nice to show the same loading spinner that is displayed when loading a spec from a URL.

Current Behavior

Currently nothing is shown.

Possible Solution

Use the same spinner that is shown when waiting for the spec to download if you are using the URL parameter.

[mckennajones]

I can probably tackle this one