Napa console updates

* Resolves this issues: https://github.com/bellycard/napa/issues/70
* Accepts an environment parameter, defaults to development if not present
* Adds `napa c` as an alias for `napa console`

Now behaves like:

```
> napa c
Rack::Shell v1.0.0 started in development environment.
> Napa.env
=> "development"

> napa c production
Rack::Shell v1.0.0 started in production environment.
> Napa.env
=> "production"

> napa console production
Rack::Shell v1.0.0 started in production environment.
> Napa.env
=> "production"

> napa c test
Rack::Shell v1.0.0 started in test environment.
> Napa.env
=> "test"

etc...
```

@skwp @jdoconnor