Books can now override any easybook global parameter

You can now override any **easybook** parameter easily in your books. Create a
new `easybook` section in your `config.yml` book and define the new value of
each parameter under the `parameters` key:

```yaml
easybook:
    parameters:
        kindlegen.command_options: '-c0 -gif verbose'
        kindlegen.path:            '/path/to/utils/KindleGen/kindlegen'
        publishing.dir.output:     '/my/path/for/books/my-book'
        slugger.options:
            separator:             '_'

book:
    title:            easybook documentation
    author:           Javier Eguiluz
    edition:          First edition
    language:         en
    publication_date: ~
    # ...
```

You can override any `src/Easybook/DependencyInjection/Application.php` parameter,
as explained in the updated chapter 3 of the documentation.