This is the commands primary method and raison detre. It prints the
applications usage text (if any) followed by a sorted listing of the
applications commands and their abstracts.
The commands are printed in sorted groups (created by sort_commands); each
group is set off by blank lines.
my @sorted = $cmd->sort_commands(@unsorted);
This method orders the list of commands into groups which it returns as a list of
arrayrefs, and optional group header strings.
By default, the first group is for the help and commands commands, and all
other commands are in the second group.
This method can be overridden by implementing the commands_groups method in
your application base clase.