Some servers use custom IRC bots with custom commands for registering to
specific services after connection.
This adds support for setting custom raw IRC messages, that will be
sent after registering to a network.
It also adds support for a custom flag.Value type for string
slice flags (flags taking several string values).
The database is now initialized automatically on first run. The schema
version is stored in SQLite's user_version special field. Migrations are
stored in an array and applied based on the schema version.
We now store SASL credentials in the database and automatically populate
them on NickServ REGISTER/IDENTIFY.
References: https://todo.sr.ht/~emersion/jounce/10
