Textfield onBlur / onFocus handlers do not repond in Firefox

[debois]

This is a regression caused by the fix to #166. We'll correct it once #179 is merged.

However, it doesn't seem we can fix this without bumping the major version, so this is unlikely to be fixed in v7.

[vipentti]

Temporary fix to enable these on Firefox is to include a polyfill.

One could use polyfill.io for example

<script src="https://cdn.polyfill.io/v2/polyfill.js?features=Event.focusin"></script>

or https://github.com/paulcpederson/focusin

[debois]

Oh, cool! Can you update the demo to check if we're on Firefox and include the polyfill; and mention it in the docs for onBlur / onFocus? (No need for a PR.)

[vipentti]

This was fixed (for now) in e533afb