default template that works for audio w/o http reference

[ultrasaurus]

I think ideally for EPUB we should have a default template that doesn't require a web request (even if that does actually work)

Initially I was thinking we would just use the circle-play icon from FontAwesome and some custom JS -- didn't even realize there was a whole HTML component library in FontAwesome!

Found this CSS example: https://codepen.io/terrytongcc/pen/YXOqLW -- I'm not really wild about the rollover glow effect, but it illustrates how easy it is to draw a circle and a triangle these days :)