Add functions to add environment variables

This is my (newbie) attempt at some functions that add an environment variable to `/etc/environment`.

It lacks tests because I don't know how to do that.  However, I have tested manually (with `python -i mcv/env.py`) and it works fine (and idempotently).

One caveat: I'm assuming that the python process running this code will have write access to `/etc/environment`.  I don't know if that's a valid assumption, but there it is.