Fix for single-slack in #9 broke multi-slack support

[jwm]

I think 6f989a6 breaks multiple server support, since init() will match all servers, but only store the server tag for the last one found. Also, slack team hostnames can include dashes, which \w won't match.

I like a slight variation (untested) on @mikegrb's suggestion:

my $svrre = qr/\.irc\.slack\.com$/;
my ($script_filename, undef, undef) = fileparse(__FILE__, '.pl');
if ($script_filename ne 'slack') {
    $script_filename =~ s/^slack_//;
    $IRSSI{name} = $script_filename;
    $svrre = qr/$script_filename\.irc\.slack\.com/;
}

That way, you can have slack.pl, with slack_team1.pl, slack_team2.pl, etc. symlinked to it.