#38 Don't welcome underscore-appended nicknames

Sometimes reconnections etc. cause a user, say janedoe, to connect with nickname janedoe_ or even janedoe__. IRC clients automatically choose these alternative nicknames IIRC. There's also janedoe1, janedoe2 etc. but the underscores seem to be much more common.

When that happens, the bot welcomes the new user even though it's someone already known by their regular no-underscore nickname. A bit annoying and spammy.

Suggestion: Whenever user X_ joins the channel, where X is the nickname characters before the last, the bot should check if it already knows nickname X. If it does, don't welcome nickname X_.

Done. Not tested yet, reopen if needed.