Display audio download progress

As higher order Ambisonics files are big in size, I want the user to see the download progress.

Maybe the function `BufferList.prototype._launchAsyncLoadTaskXHR` has to be extended for this.

I added the following lines right before `xhr.send();` (line 282 in omnitone.esm.js at the time of writing)

```
xhr.onprogress = function(event) {
if (event.lengthComputable) {
Utils.log(`Received ${event.loaded} of ${event.total} bytes`);
} else {
Utils.log(`Received ${event.loaded} bytes`); // no Content-Length
}
};
```

The result is a console display of the desired values.

Has anybody advice on how to read `event.loaded` and `event.total` outside of the Omnitone library?

Thank you in advance!

Start in omnitone.esm.js at BufferList.prototype._launchAsyncLoadTaskXHR and inspect how the XMLHttpRequest is created and used. Determine how progress values from xhr.onprogress could be made available outside the library rather than only logged; done means callers can access download progress while the audio is loading.

Written by the indexing model from the issue text.