New Git-to-HTML command

[Chris Warburton]

We need a nice way to turn a (bare) git repo into a set of static HTML
files. Currently, the git2html script can do this, but there are
problems:

 - git2html pages require a lot of disk space, since they contain the
   content of every file, formatted contents (e.g. including line
   numbers), diffs with parents, etc. for every commit on every
   branch. This is not sustainable.
 - It's straightforward enough to limit git2html to the top commit,
   which we've already done.
 - Another change is to splice the contents of any README into the
   resulting git2html index page.
 - It would be nice to have a browseable archive of Artemis
   issues. These are stored in .issues, and are in maildir format.
 - MHonArc can create a set of HTML pages from maildir.
 - Artemis issues are tracked in the repo (that's the point), but we
   don't need to provide the full history as HTML; since we only append
   to the issues (or mutate them to closed), we only need those from
   HEAD.
 - Resulting combination of git2html and mhonarc's artemis pages should
   be standalone and relativised, so it'll work e.g. via file:// or IPFS