Add `env` command

Implement: terlar/fry#29

You can now do something like this:
```
fry env > .praxrc
```

```
fry env ruby-2.1.2 > .praxrc
```

It will either output:
```
export PATH="$(fish -c 'fry current --path'):$PATH"
```

Or explicitly:
```
export PATH="/opt/rubies/ruby-2.1.2/bin:$PATH"
```