Create a new command for listing the set of .env files created with envman

We need to create a new command for listing out the named `.env` files created by `envman`.

Currently, the simplest way to do this is by running the following:

```
$ ls ~/envs
```

However, it's possible to make this more convenient by having a built-in command (e.g. `list` or `ls`) that displays this list for us.