Add option to specify a working directory for grunt.util.spawn

Fantastic plugin!  I've been using this for a while, and love it.  However, I've run across several people wishing to use grunt-concurrent from a different directory, so I thought I'd contribute to the project.

This PR adds an option to specify a working directory for grunt, in case the process.cwd() is different than the Gruntfile location.  This is especially helpful when Grunt is installed in a subdirectory of the project, but needs to modify files in sibling directories.