Add legend file for id mapping

See AT-563
I would need to add the LB tag values to the output of `Genome::Model::Tools::Sam::ListReadGroups` which wouldn't be a problem but we should discuss what the format of the output should be. The least invasive would probably be to add a second output `libraries` that is an array, ordered the same as the corresponding `read_groups` array. I would prefer a named hash of sorts but this will break things wherever this module is used. Thoughts?