Match IRC-style mentions at beginning of message

[rdb]

On my IRC channel I've noticed people expect to be able to address Discord users like they usually do on IRC, in one of these two forms:

<ircuser1> discorduser: yes, I agree
<ircuser2> discorduser, I also agree with that

This PR makes it possible by extending the mention regex to transform a discorduser: or discorduser, at the beginning of a message into @discorduser.

[coveralls]

Coverage increased (+0.009%) to 97.085% when pulling ba505ed on rdb:patch-1 into 3087661 on reactiflux:master.

[ekmartin]

This seems like a good idea to me. It would be helpful if you could try adding a test or two as well!

[rdb]

Added unit tests; looks like the checks passed.

[ekmartin]

Excellent, thanks!