Do not apply polyfill by default

[aduponchel]

Hi,

In some applications String.prototype is sealed for security reason.
In that context, define a new property on String.prototype will throw.
It could be great if polyfill could be applied only when using an explicit argument or if you could provide a way to use unorm with a sealed String.prototype

Thanks
Regards

[walling]

Hi, thanks for your request. I gladly accept PRs for this addition. Best regards