Added new field Dir.

[croqaz]

The Dir field is vital, because we don't have access to the exec.Command inside the run function, so if we need to execute the call in another working directory, currently, we can't.

I'm open to sugestions and critics 😸

[coveralls]

Coverage decreased (-99.6%) to 0.0% when pulling ef5dd7b on ShinyTrinkets:master into b84ca4f on go-cmd:master.