config: add sanity assert that files are opened as binary
This helps with some debugging in Python 3, and shouldn't hurt
anything in Python 2. The unusual construction using getattr is done
so that StringIO/BytesIO instances can be used as well as real files.
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.