#156 Per-channel header & footer

[Shish]

I'm doing stats for serveral channels, and I want a
very slightly customised header for each; each one has
a generated activity image, linked to by
"/graph.php?channel=foo". Rather than make a separate
header.txt for each, I've patched HTMLGenerator to
replace {{foo}} with $self->{cfg}->{foo}

Example output --> http://ircarchive.ath.cx/