Added application scaffold generator

This PR add a generator to the belly_platform gem that allows us to do something like this:

```
belly_platform new foo_service
```

and generate a scaffold for a new app that looks like this:

```
Creating File: foo_service/app/apis/.keep
Creating File: foo_service/app/models/.keep
Creating File: foo_service/config/database-example.yml
Creating File: foo_service/config/initializers/active_record.rb
Creating File: foo_service/config/middleware/honeybadger.rb
Creating File: foo_service/db/migrate/migration.template
Creating File: foo_service/lib/tasks/db.rake
Creating File: foo_service/log/.keep
Creating File: foo_service/public/.keep
Creating File: foo_service/spec/spec_helper.rb
Creating File: foo_service/tmp/.keep
Creating File: foo_service/.env
Creating File: foo_service/.env.test
Creating File: foo_service/.gitignore
Creating File: foo_service/app.rb
Creating File: foo_service/config.ru
Creating File: foo_service/console
Creating File: foo_service/Gemfile
Creating File: foo_service/Rakefile
Creating File: foo_service/README.md
```

Right now this bakes in things like ActiveRecord and Honeybadger, I'd like to make those optional via flags at some point.

Feedback?