Simplified install using brewcask and hiera-managed params

Now that [boxen/puppet-boxen](https://github.com/boxen/puppet-boxen) has added [boxen/puppet-brewcask](https://github.com/boxen/puppet-brewcask) as a core dependency and recommends using per-user hiera configuration, should we utilise these methods of installing Atom and its packages/themes?

What this PR does:
- Installs Atom through brewcask
- Ability to pass in packages and themes for easy installing through Hiera-managed params

All that's then needed to install Atom and any packages/ themes is this YAML:
```yaml
atom::packages:
  - linter

atom::themes:
  - monokai

boxen::personal::includes:
  - atom
```

Thoughts?