Blog post index working.

This one's pretty complicated.
It looks through the template for vars with the same name as a subdir.
It then gets a list of all the files in said subdir, and provides that
list to the template as the var in question.
This lets you form links to all the files in a subdirectory.
Those links will break if the file isn't built into html.

Yeah. This is pretty brittle. Works, though.