Support showing workflow status of multiple repos in one command

[alarthast]

• Currently show all and show [org] display summaries of workflow statuses for predefined groups of repos in config.py
• Might be useful to define a show-multiple job that allows the user to designate multiple repos (i.e. effectively creating their own group).
• Implementing this might mean that we can incorporate the currently custom check-links command into show.
• Slack conversation 1
• Slack conversation 2