Have pt-query-digest option for choosing maximum hostname length.

Currently, pt-query-digest trims hostnames to 10 charachters:

my $print_str;
if ( $str =~ m/(?:\d+\.){3}\d+/ ) {
$print_str = $str; # Do not shorten IP addresses.
}
elsif ( length $str > MAX_STRING_LENGTH ) {
$print_str = substr($str, 0, MAX_STRING_LENGTH) . '...';
}
else {
$print_str = $str;
}

It would be nice to have an argument added for this, so that the user can choose maximum length to be used, without having to manually tweaking the code (the MAX_STRING_LENGTH constant).