The semantics of os.listdir() here are better than Path.iterdir(), since
the former provides us paths relative to input_dir which we copy
directly over to output_dir. The pathlib version returns the full path
to input_dir/file and then we'd have to get file.name outselves.
* move sys import to where it's used
* remove commented-out code
* change parser -> config in read_config() for slightly better readability
* change meta_dict -> meta and file_contents -> contents in
handle_file()
* push handle_frontmatter() back into handle_file(). likely to pull
this back out again if we decide the format is handy for keeping
default values around in templates, but we'll cross that bridge later
