Generate ERFA wrappers (with modifications to astropy-helpers)

Because code-based discussions are better than code-less discussions! :smile:

This is definitely WIP, but a proof-of-concept of the idea of adding a ``preprocess_source`` function to ``setup_package.py``. Of course, we don't want all the stdout cluttering, we also may not want to always generate the output if it is up-to-date, etc. But I just thought this would be a good starting point to discuss how  we could proceed.

This also doesn't do anything (yet) about @mdboom's suggestion of generating only things that require Cython or Jinja for tarballs.

The changes in astropy-helpers are: https://github.com/astrofrog/astropy-helpers/compare/preprocess

cc @embray @mdboom @taldcroft @eteq @jwoillez @mhvk 
yes, in favour of `importlib`: https://docs.python.org/3/library/imp.html
`importlib` is available from python 3.1 onwards (so all our python3), so one could make a switch between python2 and 3 here. But I think it is fine to do this in a subsequent PR.