Support specifying repositories and their profiles in the configuration file

[jwodder]

apply and its configuration file should support the following uses:

• Specifying the profile to use for a repository (when --profile isn't given on the command line) in the configuration file
• Specifying a collection of repositories to operate on when some option (--all-repos?) is given on the command line

Possible configuration structure:

[repositories]
"owner/name" = "default"
"octocat/hello-world" = "custom"

With this structure, --all-repos operates exactly on those repos used as keys in [repositories], which means that, in order to include a repo that uses the default profile, the profile has to be given explicitly.