Parameter definitions have been added

Hi @jjanvier 

We have been using the bundle at commit #1436ffee093c9ccf13abfa7f0c86417f7d09cc26
Trying to upgrade it to latest version met new Sync* commands, which depend on set of `jjanvier_crowdin.github.*` parameters. These params are created from `github` section of the bundle config.
And since we don't use them, we have to fill this config section with stubs. Otherwise `Symfony\Component\DependencyInjection\Exception\ParameterNotFoundException` being thrown.
I'm also now allowed to set them to `null` because of bundle `Configuration`.

This pull request allows to keep `config.yml` of the project more neet.
Compare:
```yml
jjanvier_crowdin:
    crowdin:
        api_key: %crowdin_api_key%
        project_identifier: %crowdin_project_identifier%
    github:
        username: "something_useless"
        email: "something_useless"
        token: "something_useless"
        organization: "something_useless"
        project: "something_useless"
```
And:
```yml
jjanvier_crowdin:
    crowdin:
        api_key: %crowdin_api_key%
        project_identifier: %crowdin_project_identifier%
```

Thanks in advance