AUTHENTICATE: support IRCv3.1 SASL PLAIN Authentication

[zachdecook]

Tested with senpai

We may want to add this config to the readme, it seems like enough features are working, but I haven't tested it too much.

address localhost:6666
password-cmd autotoken -c /snap/bin/chromium myworkplace [email protected] SECRETPASSWORD
nickname put.anything.here
tls false

Implementation reference

• https://ircv3.net/specs/extensions/sasl-3.1
• https://www.rfc-editor.org/rfc/rfc4616