Allow passing arguments

[megawubs]

I want to be able to pass arguments to the phpunit process. For example, I would like to pass

-c ohter-phpunit.xml

[olimorris]

Great suggestion and I'd be more than happy to accept a PR for this.

[sebby]

Inside the phpunit_cmd, just like that ?

phpunit_cmd = function()
   return "phpunit -c core/phpunit.xml"
 end,

Isn't possible inside the root_files parameter ?