Add support for additional VR viewers parameters to be passed in

[wan-hellopupil]

Allows for other viewer parameters to be used. The goal is to then be able to do this:

var polyfill = new WebVRPolyfill({
    ADDITIONAL_VIEWERS: [{
        id: 'Some_different_cardboard_viewer',
        label: 'My Best Viewer',
        fov: 50,
        interLensDistance: 0.053,
        baselineLensDistance: 0.035,
        screenLensDistance: 0.053,
        distortionCoefficients: [0.15, 0.11],
    }],
    DEFAULT_VIEWER: 'Some_different_cardboard_viewer',
});

and have the new viewer present in the list when clicking on the gear icon.

[wan-hellopupil]

(this would also need the following change in the webvr-polyfill to work):

       BUFFER_SCALE:                 this.config.BUFFER_SCALE,
       DIRTY_SUBMIT_FRAME_BINDINGS:  this.config.DIRTY_SUBMIT_FRAME_BINDINGS,
+      ADDITIONAL_VIEWERS:           this.config.ADDITIONAL_VIEWERS,
+      DEFAULT_VIEWER:               this.config.DEFAULT_VIEWER,
     });

     this.polyfillDisplays.push(vrDisplay);

[wan-hellopupil]

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• [x] This PR does not contain built changes in dist/\* (required)
• [x] This PR only contains one commit (required)

[jsantell]

Thanks for sending a PR, @wan-hellopupil! Sorry it's taken me a bit to respond, I'll look over this and do some testing over the next week and get this into WebVR polyfill, this is a great feature that I know a handful of folks interested in. Thank you!