add change-pgrp option (default true)

I want to use foreman in the Procfile.

    # Procfile
    foreman:   bundle exec foreman start -f Procfile.internal

In the child foreman process, I don't want to change its process group because the parent foreman process will send signal to the parent's process group. So I added `--change-pgrp` option. The default value is `true`. If someone wants to use foreman in Procfile, he or she can use `--no-change-pgrp` option in the Procfile.

    # Procfile
    foreman:   bundle exec foreman start -f Procfile.internal --no-change-prgp