More precise nick alerts (#60)

Highlight when a nick matches _and_ the match is surrounded by word
boundaries. This way a user called "foo" won't get highlighted when
someone writes "foobar", but foo: foo, "foo" etc. still trigger the
highlight as expected. Furthermore, since newbies often write nicks
by hand instead of using auto-completion, the nick matching is still
kept case insensitive for now. We can easily turn it case sensitive
later if people complain about false alarms...