Consider alternative filenames than Procfile

[ejholmes]

Switching to the extended Procfile can cause negative development side effects, if you are, for example, using foreman, which won't know how to read the extended Procfile format. An easy workaround is to just create a Procfile.dev file and use the -f flag, but maybe we could support some fallback filenames instead so that development remains unchanged:

Procfile.empire -> Procfile -> Docker CMD

Thoughts?