Re: [me-media-timed-events] Add recommendation for events with unknown end time (#39)

@mavgit Thanks for the clear use cases [above](https://github.com/w3c/me-media-timed-events/issues/39#issuecomment-494596204), which have made me realise that these can already be supported by the API, as follows:

a. The [`TextTrackCueList`](https://html.spec.whatwg.org/multipage/media.html#texttrackcuelist) is an existing interface, so cues can be read from the [`TextTrack`](https://html.spec.whatwg.org/multipage/media.html#texttrack) object.

b. The [`TextTrackCue`](https://html.spec.whatwg.org/multipage/media.html#texttrackcue) object exposes the `endTime` attribute which [can be set](https://html.spec.whatwg.org/multipage/media.html#dom-texttrackcue-endtime).

c. Hence, cues can be updated by removing, updating and then adding them back to the list. For example:
```javascript
function updateCueEnd(textTrack, cueIndex, endTime) {
  // read cue to update
  var cue = textTrack.cues[cueIndex];

  // remove cue
  textTrack.removeCue(cue);

  // update end time
  cue.endTime = endTime;

  // add updated cue
  textTrack.addCue(cue);
}
```

[Best practices for metadata text tracks](https://html.spec.whatwg.org/multipage/media.html#best-practices-for-metadata-text-tracks) describes a recommendation for handling scoring for a sports event and considers the live streaming case, which offers another good use case for updating media timed events. At the start of the match the score is '0-0', which could change to either '1-0' or '0-1' at an unknown time in the future.

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