Make the bootstrap a global command, not required in project

From a quick investigation:

`composer require humanmade/reusable-features`:

1. Adds it to your composer.json + lock as a permanent dependency
2. Drops the shell script and a features.txt catalog files into vendor/humanmade/reusable-features/

This would be better implemented as a global command that can be run independent of a project, so that there's no unneccesary project dependency being added.

eg:

```
composer global require humanmade/reusable-features
reusable-features # runs anywhere, in any project

#or just
curl -fsSL https://raw.githubusercontent.com/humanmade/reusable-features/main/bin/bootstrap.sh | bash
```