Allow for specifying processes to not run

[deecewan]

This is useful in the case where you want to not run one or two process, but keep the rest of the processes running as usual. Inverse of the OVERMIND_PROCESSES flag. Takes precedence over that flag.

My particular use case is building a react-native app, and needing to run/not run some of the processes per OS. One option is to maintain a list of every process, and add the necessary ones for each OS. The other is to turn off the unused/unwanted processes for each OS.

This is a more sustainable solution as I won't have to go in and update the master list every time the Procfile changes.

Closes #92

I have "tested" this by running the newly built binary against another project that uses overmind.

[deecewan]

I guess another way to do this would be in the more foreman style where I could specify OVERMIND_PROCESSES=service_a=1,service_b=0. If there are no numbers, it defaults to a 1. That would also prevent the need to have another flag, and would remove ambiguity around what happens when you specify the same process in both PROCESSES and IGNORE_PROCESSES.

[deecewan]

Actually, this can be done with the OVERMIND_FORMATION flag 👍 I didn't realise that flag existed because it's not in the README 😭

[deecewan]

Got some feedback that this might still be valuable as an easier way to prevent a process running 1-off, with a simple -x procname instead of having to do -m "all=1,procname=0". So, will ask @DarthSim to make a call on whether to add this or not.

[DarthSim]

Hey @deecewan! The feature is pretty simple and well implemented, so I have no reason not to merge this. LGTM. Thanks for your contribution!

[matthutchinson]

👏 I was just about to suggest adding something like this 👍