[cli] Port the email command to Core.Command.
Start porting the harness to Core.
Add harness import.
Add '_build' to .gitignore.
Generalize the build function to process ocamlbuild exit code.
Move test command functionality to its own module.
Change the exit-code handling to an optional argument.
Finish helper functions for harness.
Add function to get exit code of build.
Move the rubric functionality to its own module.
Add function to get subdirectories.
Finish the initial port of the harness command.
Merge branch 'command' of github.com:bkc39/tools into command