add autorun option to prevent automatic loading of minitest/autorun

Hi. I've got a project that uses Celluloid which wants to shut itself down via a ```Kernel.at_exit``` hook. Also, ```minitest/autorun``` likes to run it's tests via a ```Kernel.at_exit``` hook. When guard-minitest is involved, it takes control of when ```minitest/autorun``` is loaded out of the hands of the developer by forcing it to be required first. This causes Celluloid to shutdown before the specs are run. 

I'd like to have the option to decide when that happens: in my ```spec_helper.rb``` after I've done a ```require 'celluloid'```. I've added an ```autorun``` option to meet these needs, and it defaults to true to match the current behaviour.