Add support for auto-detecting publishable key?

[Firehed]

While this probably won't be useful for NPM-type apps, some application templating systems will make it easier to dump a value on the page somewhere and handle it through the client. There are a couple of paths that could probably improve setup:

<script>
const SNAPAUTH_PUBLISHABLE_KEY = '(set by backend rendering)'
// ... probably in a different script
const snapAuth = new SnapAuth.SDK()
</script>

and

<script src="https://cdn.snapauth.app/js/1.2.3.js" data-key="pubkey_abc123"></script>

Exact mechanics TBD, but I found some of the setup mechanics a little clumsy when making a demo with Laravel (specifically with getting the right envvars into the JS)