Parallelize locally and allow split repositories

[alex-pinkus]

Adding functionality to specify "part X of Y" of a repository. This
allows two things to happen:

1. In CI, that repository will get split across multiple Github Actions
   hosts, each one having less work to do
2. Running locally, the script will spin off each validation check as
   its own process and they will all return more quickly.

This is necessary to add firefox-ios as a top repository since it has
657 swift files, which would otherwise take 5 minutes on CI, and I'm not
ready to wait that long.

Fixes #49