Deployment with Bash command

Using this piece of YAML:
```
deploy:
	bash:
		command: 'bundle exec cap production deploy'
```

Will run Capistrano and deploy your app to production.
You can use any other deployment tool instead of Capistrano.

It's not documented well now and I'm ready to add a section into https://github.com/drone/docs/blob/master/deployment.rst as soon as you merge this.