Be able to do a "dry run" to see which projects would be evaluated

[mccurdyc]

What would you like to be added:

• would like to be able to see the repos that neighbor is going to use
• maybe just a --list_repos flag
• this is particularly useful before you run an external command that would manipulate the repo (e.g., PR, etc.)
• primarily to nail down the github search query before performing any external command
• it is important that this list of repos gets stored somewhere (in a .data dir or something) because the repos on github that suffice the query could change from run to run

Why is this needed:

• guarantee that you are running an external command on the set or repos that you expected

Definition of done

• when i can run --list_repos and the repos are listed without running an external command