Add biber extension

[akerbos]

Apparently, bibtex (together with natbib) is slowly being made obsolete by biber (in combination with biblatex). Therefore, we should have an extension for biber.

Key question/challenge: can we tell which of bibtex and biber has to be executed? This is not an issue with natbib but biblatex supports both. Does it tell us which it expects?

[akerbos]

Good news. With backend=bibtex, biblatex may write something like this to jobname.aux:

\bibstyle{biblatex}
\bibdata{test-blx,<database>}
\citation{biblatex-control}
\citation{<some-ref>}

With backend=biber, on the other hand, it's:

\abx@aux@sortscheme{nty}
\abx@aux@cite{<some-ref>}

[akerbos]

Better news: the current bibtex extension checks the aux file for \bibdata commands; these are not used for biber, so it does not execute on biblatex with biber option, but does on biblatex with bibtex option. Fine!

With biber target, however, biblatex writes a file jobname.bcf which we can check for.