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.
Thus using the tools above I feel like the shorter/idiomatic way to construct this would be:

```python
lines = envfile.readlines()
matches = [re.match(r'(\w+)=([^#]+)', line) for line in lines]
return [[m.group(1), m.group(2).strip()] for m in matches if m]
```

Can also optionally just convert it into a dictionary using a dictionary comprehension:

```python
{m.group(1):m.group(2).strip() for m ...}
```