Slack users mentioned in IRC don't get @ prepended

[Jaykul]

Expected:

When someone in IRC mentions a slack user, it should show up as a @-mention in slack (so it alerts their client).

Observed:

IRC users do not add @ to the front of names when they mention other users, so the bot should search for user names and prepend @ to them -- at least if the name is the first thing in the message.

Repro

Check out the difference between how these messages show up in slack when sent from IRC:
Jaykul: check this out!
@Jaykul: check this out!

We want them both to show up the same (with the "@" on the front).

[Jaykul]

This might be addressed best by training people to write @, but some IRC clients don't like that and won't tab-expand nicks in that case (even if you're logged in on both sides).