Limited the number of children processes created

First of all, thanks for taking the time to build this plugin, it just saved me a lot of trouble!

I've modified the script. I was running it with more than ten sub-projects (not ideal, but it's a very specific project with complex templates). Each time I rebuilt the full project with grunt-subgrunt it was killing my system with only two cores and lots of processes trying to use the whole cpu. 

Merit from this code comes from [grunt-concurrent](https://github.com/sindresorhus/grunt-concurrent/)