Fix crash in start when all processes are quiet

Specifying all processes in a Procfile as --quiet, caused `max` to be
called with no arguments, which crashes.
command: Remove metaclass magic

This commit removes all the metaclass magic in honcho.command in favour
of a set of simple nested argparse parsers.
command,environ: Move env parsing into own module
command: Make Procfile and env loading respect app_root
command: Change default log format to a more helpful one
test: Treat each procfile parse test separately