you've modified vendored files, could you please make your PR against upstream: https://github.com/42wim/mm-go-irckit ? (I know it's annoying, am planning to add it back to matterircd instead of stand-alone lib)
matterircd supports slack aswel as mattermost, so the PASS should work for both. Maybe detect if the pass is a slack token, or an extra argument to specify mattermost or slack
this allows to just pass the mattermost credentials as the server password in the irc client