With pass extensions now included in the Debian repos (and the repos
of other major GNU/Linux distributions, it was time to redo the
README.

 * Enable git2changelog to emit version strings
 * Adjust Makefile so it uses git2changelog's version string
 * Handle triplet tags, not just major/minor version tags
 * Prepend tags with 'v', per GitHub's practice

Versions were not getting set properly in the absence of proper tags.

Enough time has passed that pass-git-svn is in the Debian repos.

KFogel emailed to note the complaint and while it's not a problem,
there should be no complaint if there is no problem.

* Use DEBEMAIL and DEBFULLNAME from environment if set
* Add targets for distclean and README.mdwn