Allow the OmniAuth provider args parameter to pass through

The gitlab.yml.example has said for a long time that you can add an additional 'args' parameter to configure an OmniAuth provider, however this was never passed through to omniauth. The additional parameters are required to correctly configure Google OAuth2 and many other providers.

Configuration example that has been in gitlab.yml.example since the original omniauth PR a64aff2f1c1ddc77b00211489d74fbc23c0c2fa2 and after this PR it will work.

```yaml
  omniauth:
    ...

    providers:
       - { name: 'google_oauth2', app_id: 'YOUR APP ID',
           app_secret: 'YOUR APP SECRET',
           args: { access_type: 'offline', approval_prompt: '' } }
```