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.
If the above is done as a dictionary, you get auto key presence checking for free:

```python
>>> m = {'foo': 42, 'bar': 84}
>>> 'foo' in m
True
>>> 'bar' in m
True
>>> 'baz' in m
False
```