Allow to add other environment variables

[Hanusz Leszek]

Currently, it is only possible to change the port and node_env environment variables.

I would like to set another environment variable.
In my case it is LIVERELOAD_PORT to be able to change the livereload port from the Gruntfile.js

[Paul Pflugradt]

Actually the whole process.env is passed to the child here

You could probably change the env programmatically with this

[Hanusz Leszek]

Thanks.