Unbreak the render() API, adapt to changes in formulare-bfinv.de, somewhat adaptive waiting for pdf download

The YAML file must contain at least the following fields:

Vorname: Hans
Nachname: Acker
Adresse:
  Straße: Hack Hack Hack 1234
  PLZ: 31337
  Ort: Hacktown

I've added a custom loader because the C3PB member data contains a weird
YAML tag that needs to be handled explicitly: !file(konto.yml), which is
used for including bank account data.