Merge settings with file configuration

This makes it possible to combine settings in a configuration file with settings from a PasteDeploy .ini-file or in-Python defaults. Settings in the configuration file will override others.

Example usages:

```python
# app.py
app = config.make_wsgi_app()
return make_appenlight_middleware(app, {'appenlight.logging': True})
```

```config
[app:main]
appenlight.api_key = 123456
appenlight.config_path = %(here)s/etc/appenlight.ini
```