expose command prefix in `irc.Config`

[theaog]

expose the command prefix variable in the Config struct, the current OS ENV solution feels like a cheat, plus it doesn't apply for plugins and is not documented.

let me know if I could send a PR w/ the change.

[fabioxgn]

Hi, @theaog I'd really appreciate it if you could make this change! Thanks for the interest in helping the project.