[w3c/manifest] Ability to detect if the current document is part of an "installed" web app (#454)

Forked from #417 

It would be very valuable to have an API that allowed a website to know what manifest the user agent has loaded for the current scope (if any).

## PROPOSAL

Add an API to get the current manifest

Something like:

```javascript
var manifest = navigator.document.getManifest();
if(manifest){
  var startUrl = manifest.start_url;
}
else {
//No manifest loaded
}
```

## Questions

 1. Where is the right place for the API (navigator, document, other)?
 1. What happens in the case where two manifests have overlapping scope and the current URL is in that shared scope space?
 1. What properties/methods should be available on the Manifest object beyond the data members of the original JSON file? (Install date?, Install URL?, Update method?)

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