Allow for an ignore "Defaults" block in the YAML

I'm managing multiple apps, and I want to be DRY, so I need a place to define some YAML nodes to then reference:

```yaml
defaults:                      
  addons: &addons         
    pgbackups: plus            
    sendgrid: starter          
    papertrail: choklad        
    heroku-postgresql: dev     
  config: &config         
    BUNDLE_WITHOUT: "development:test"
    AWS_ACCESS_KEY_ID: 'key'
    AWS_SECRET_ACCESS_KEY: 'secret'

app1:
  stack: cedar
  app: awesome-app-the-first
  addons:
    <<: *addons
  config: 
    <<: *config
    DOMAIN: "awesome-app-the-first.herokuapp.com"
    AWS_BUCKET: 'bucket'
```