Cleaner conversion of rST files

[drammock]

This is an alternative approach to #33 to converting conventions.rst into HTML. It uses rst2html5 to do the conversion, plus a small python post-processor script to force the highest HTML heading level to be h2 (to match the other pages on phoible.org). A Makefile recipe is provided to chain these operations.

currently it only writes the body content, which I think will make it easier to integrate into the main phoible.org webapp, at the expense of generating something pretty for phoible.github.io (which is slated to become a redirect to phoible.org anyway). It's possible to change _template.html to include html, head etc: see b3e31e9 for an example.

TODO:

• do similar cleanup with the FAQ page processing (e.g., add a recipe for it to the Makefile; don't depend on presence of RStudio)
• coordinate with @xrotwang to get this into the right place within https://github.com/clld/phoible

closes #33