`bin/setup` script to bootstrap applications. [WIP]

This is a first draft to collect feedback.

### About

While this script does not perform much work out of the box, the idea is to have a starting point to add your own setup code. Having an automated way to bootstrap your application saves not only time but also serves as verifiable documentation. By default the script does:
* `bundle install`
* `bin/rake db:setup`

To encourage users to modify it, I included some suggestions what you might want to do.

### Work

- [ ] document, where should this convention be documented?