Add support for negative instances count

[shaharmor]

Hi,

I think it would be nice to have an option for the instances option to be negative (for example -2), which will then start N-2 instances, N being the cpu count.

So on an 8 core server, i could set instances to "-2", and it will spawn 6 instances, and on a 4 core server it will spawn two.

What do you think?

[jshkurti]

Here we go ;)
npm i -g git://github.com/Unitech/PM2#development

[shaharmor]

Awersome. Thanks a lot.