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. 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.