Cont. clean up bundled polyfills

[hartzis]

As a follow up to #26. Option 2 implementation tracking with this issue 😸:

• just transpile source code to es5+/es6? and not polyfill.
  • Update the readme to make note of which features the user should polyfill if needed

After updating after pr #27 to react-google-recaptcha@^0.13.0 which uses this library at react-async-script ">=0.10.0" the bundled polyfills dropped from 21kb parsed / 7.5kb gzip to 15kb parsed / 5.5kb gzip.

Great first step, but I'd argue that is still way too much, especially with browser support for es5+ at all time highs.

Those sizes above include both react-google-recaptcha and react-async-script bundled polyfills.

[hartzis]

This looks good with the new release and merging of #29 ! thanks @dozoisch !