Section grouping

This implements a new feature called Section Grouping, discussed on issue #25.

The solution is quite simple, and could be reviewed to later refactoring.

All you have to do is declare the same `section` property to two or more actions, and they will be displayed grouped, just the way resources are.

This has precedence over the `resource`, but that can be easily changed.

The default `section` is `'none'`.