Include keydown event when unlocking audio

[abrom]

To allow users with accessibility issues use our site we needed to support "unlocking" Howler using non-click/touch based events. The keydown event seemed the most appropriate.

I noticed there have been other discussions that have skirted around expanding on the existing list of events that trigger the unlock but I couldn't find anything concrete.

@oswaldofreitas mentioned in #1294 about using:

  'click',
  'contextmenu',
  'auxclick',
  'dblclick',
  'mousedown',
  'mouseup',
  'pointerup',
  'touchend',
  'keydown',
  'keyup',

And @Jimbly appears to be listening for

touchstart, touchend, click, mousedown and keydown

https://github.com/Jimbly/howler.js/blob/master/src/howler.core.js#L433-L438

In the short term at least, adding keydown (and maybe keyup?) would help with the accessibility issues.

Great project BTW 👍

[Jimbly]

I believe "keydown" and "mousedown" were the specific one I needed to add to fix actual observed problems as well, but having more seems like it cannot hurt, since they're cleaned up as soon as any one is fired.