Add possibility to have prefixed config items override unprefixed ones

* this can be very useful for having a single project that is used for
  both "local" and "containerized" execution, where some configuration
  items should be slightly different for either (e.g. `SEARCH_HOSTS`)
* currently we have this functionality implemented in our `invenio.cfg`
  but there it is a bit awkward due to the fact that environment
  variables have not been translated into `app.config` yet