Add option to use expire_at instead of expire_duration

[joeyparis]

Add the option to pass an expire_at value to the LoginToken.encode method that overrides the global passwordless_login_within config value on a per-link basis. This does not yet implement the expire_at option for sending emails with the magic link, just the encode method for manual encoding.

Addresses #18

[abevoelker]

Thank you very much for your contribution! The option to provide an :expires_at key for token encoding¹ has been implemented for the 1.0 release, and I've credited you in the CHANGELOG. 🎉 🙇‍♂️

Footnotes

1. This logic now lives in the SignedGlobalIDTokenizer::encode and MessageEncryptorTokenizer::encode methods, now that the tokenization process is configurable ↩