Passenger compatibility 
Hello,

I've just added the rate limit to my API thanks to your module.

So I've put my API in preprod, and now I've noticed that the server doesn't start. After some investigation, I found the problem.
We use Passenger, which is not compatible with the Node Cluster module.
https://www.phusionpassenger.com/library/walkthroughs/basics/nodejs/fundamental_concepts.html#vs-cluster-module

If I'm not mistaken, this module is used for the "cluster" mode and the "PM2" mode.
Would it be possible to make these 2 modes optional or deactivatable?
