Add “config compare” command

Often times when we’re developing on multiple environments, it’s easy to forget to add or remove environment variables as we deploy newer code.

The `hanzo config compare` command tries to ease this process by providing a list of variables that are not set on all  environments. Its output is pretty simple:

```
% hanzo config compare
-----> Fetching environment variables
       heroku config -r qa
       heroku config -r staging
       heroku config -r production
-----> Comparing environment variables
       Missing variables in qa
       - GOOGLE_ANALYTICS_DOMAIN
       - GOOGLE_ANALYTICS_TRACKER
       Missing variables in staging
       - NEW_FEATURE_VAR
       Missing variables in production
       - NEW_FEATURE_VAR
```