only generate supervisor conf if num procs is -ge 1

Turns out just using the SCALE file trick referenced in #8 doesn't work. This PR allows the user to set NUM_PROCS to 0 via the SCALE file (which skips the generation of supervisor config) while retaining the standard functionality of running every process in the Procfile with the default NUM_PROCS of 1 or with whatever is set in the SCALE file.

Also removes the need for a newline at the end of the Procfile and makes matching against the process name in the SCALE file more robust.

Bonus!: You can now ship a SCALE file along side the Procfile