Feature/support for environment

Hi there,

this is the first attempt to support a step which sets environment variables for a process which will be run with `run_simple` or `run_interactive`. 

I saw that there's a method called `set_env`, but I think the library makes use of it in a different context. Therefor I implement a ` set_cmd_env`-method.

Cheers,
dg-vrnetze