Add an "Automatic" bibliography processor pref option

Add an "Automatic" bibliography processor pref option

This is now set as default. It selects biber for Biblatex (with
fall-back to first bibtex8, then bibtex, if the former is not
installed), and bibtex for BibTeX-based engines.

With this, users do not normally need to care for the processor when
they switch cite engines.