asciidoc: add odf backend, for OpenDocument output

See https://github.com/dagwieers/asciidoc-odf for more info.

This plugin has a shebang with /bin/env instead of /usr/bin/env, so update the
patchPhase to handle both cases.

WARNING/TODO: Libreoffice says "General Error. General input/output error."
when I try to open the generated .fodp files. So the odt backend works fine,
but the odp backend does not.
asciidoc: fixup for matplotlib filter (remove .old/)

The matplotlib filter comes with an .old/ directory which contains a
chart-filter.conf file that asciidoc loads. I highly doubt that is intended.