[audioplayer] Proposal to add a "when" tag option

[floriancargoet]

audioplayer tags are processed immediately but the paragraph they are associated with may appear later (because of animations or a #delay).
I believe audio should be triggered only when the paragraph is shown.
Something like this works nicely.

Tags.add("playonce", function(story, property)
{
	property = process(story, property);
	story.queue.onShow(() => {
		audio.play(story, property, property.options, false); 
	});
});

What do you think?

We may go further and anticipate people wanting to trigger it after the animation (onRendered).
I think a nice way of exposing those possibilities would be to add a "when" option.

#playonce: my_sound >> when:shown
#playonce: my_sound >> when:rendered
#playonce: my_sound >> when:now // default

We could do "rendered + 1000ms" by combining it with delay

#playonce: my_sound >> when:rendered, delay:1000

Tell me if you like it and I'll send a PR.