add IRC auth to userdb

[slingamn]

I just merged ergochat/ergo#1111, which adds support for authentication plugins via subprocess invocation. The goal in this issue is to start managing IRC authentication credentials in userdb, then get everyone using SASL. Here's a tentative plan:

1. hashbangctl should generate a strong, secret password on new account creation (@benharri was uncomfortable with this? I definitely want to hear any objections out)
2. Write a script to check this password against userdb
3. Turn off account registration in oragono, and instead configure this script (in the new accounts.auth-script config block), with autocreate enabled
4. New account creation should automatically initialize ~/.weechat/irc.conf to use SASL PLAIN with the autogenerated password. We could write a script like enable-irc-sasl that does this for existing users.