Allow session and environment to be passed via extra parameters.

This permits the following:

    extra = {}
    extra['session'] = {'user_id': 100}
    logger.exception('passing session details', extra=extra)
    extra['environment'] = {'PATH': '/bin/usr/'}
    logger.exception('passing environment details', extra=extra)

and those extra parameters will not be duplicated in the parameters view in Airbrake.