Merge in template file for Dokuwiki support

Please could `claremacrae/pandoc-templates` branch `dokuwiki` be merged into `jgm/pandoc-templates` branch `master`.

It just adds a new file, `default.dokuwiki`, which I believe will enable me to build my fork of `jgm/pandoc`, without having to edit pandoc's `.gitmodules` to point to my fork.

(I'm trying to merge the latest pandoc code into https://github.com/claremacrae/pandoc/tree/dokuwiki, to finalise my work on jgm/pandoc#386)