Start audio context

[therewasaguy]

following up on #283

This pulls in Yotam Mann's StartAudioContext.

I got a bit sidetracked looking at his new library UnmuteButton which would be another very cool solution. But it would involve upgrading to Tone 13.2, and shimming the unmute library (which uses class) to es5 or making p5.sound es6 friendly. If anyone wants to take this on that would be amazing.

In the meantime, this PR:

• adds a userStartAudio method
• adds a dependencey on StartAudioContext

To test:

• launch Chrome Canary (70+) from the terminal with the following --disable-features flags, i.e.

/Applications/Google\ Chrome\ Canary.app/Contents/MacOS/Google\ Chrome\ Canary
--disable-features=PreloadMediaEngagementData,AutoplayIgnoreWebAudio,MediaEngagementBypassAutoplayPolicies

Here's a little sketch to test—it should alert and remove the button after userStartAudio succeeds

function setup() {
  var ac = getAudioContext();

  ac.suspend().then(function() {
    var myButton = createButton('click to start audio');
    myButton.position(0, 0);

    userStartAudio(myButton, function() {
      alert(ac.state);
      myButton.remove();
    });
  });
}