readers: Add 'static' to the base Reader's extensions

Instead of just being a base class, we can use it to parse static
files.  It won't actually do any parsing, but we will get path
metadata extraction from read_file, and this will make the
StaticGenerator implementation simpler.
generators: Use the static format to read Static metadata

This cuts down on the remaining difference between static files and
articles/pages.  The main difference is that path-based metadata is
now parsed for static content.
Merge pull request #794 from wking/static-reader

Use the static format to read Static metadata