No IE11 support because of for...of loops

[tansongyang]

Issue

Because of for...of loops, this component does not support IE11. (And possibly other reasons—I mention the loops because they are what caused the errors in my case). You will get an error saying that Symbol is undefined.

Proposal

I see a couple of options here:

1. Make it more explicit in the docs that this is a requirement. As of this writing, the docs only mention polyfills for Promise and fetch
2. Replace the loops with something like Array.prototype.forEach.

It's up to you. I'd be happy to do a PR either way.

[cookpete]

From a quick test, it looks like it isn't just Symbol that requires a polyfill, and together with the fact that IE is down to about 3% market share, I'm more inclined towards suggesting using a polyfill in the readme, rather than changing the library code in an attempt to support a dying browser.

After updating the library to remove for..of loops I started getting a pretty useless error in IE around this line:

Object doesn't support this action

Then after adding babel-polyfill the library seems to work, although videos were failing to play after loading. The youtube player demo page also seems to not play on IE11, which is slightly comforting.

[tansongyang]

OK. What is your preference on how to update the README? I might have some time to investigate the specific things that need polyfilling in IE11. Or we could just say "If you use IE11, you babel-polyfill".

[cookpete]

I think most people targeting IE11 will be using babel-polyfill anyway, so I've just added a generic hint to the readme. I'll add babel-polyfill to the demo build too.