Refactored Configurator to ease addition of new steps

| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | N/A
| Fixed tickets | #90
| License       | MIT

This PR gives the possibility to easily add new steps to the web configurator, with priority.

```yaml
services:
    my_custom_step:
        class: %my_custom_step.class%
        tags:
            - { name: webconfigurator.step, priority: 20 }
```

It would be nice to have it in 2.3 branch :smiley: