feat: support browser extensions

[raynirola]

This PR adds support for browser extensions. No breaking change, optional mode parameter set to browser by default. Fixes #45

[dagnelies]

That looks quite odd and seem to differ from what the specs says. But it would not be the first time that reality differs from the specs. 🙄

That said, I'd rather allow people to customize/override rp.id/name instead. It's more intuitive, simpler to understand and would also solve the subdomains and iframes use cases.

[dagnelies]

Solved here instead: 1c005db

You can now override rp:{id:..., name:...} in the registration options