Support multiple repositories in one command

I'd love to see the following format supported:

ghi list -- Paws/Issues elliottcable/Paws.js

… to show a combined list, or simply two sequential lists, of all of the matching issues between both of those repositories.

Similarly, I should be able to store multiple entries in the configuration variable to save me all that typing:

git config ghi.repo "Paws/Issues elliottcable/Paws.js"