Added script to create template post.

This commit adds the `create-post.py` script which allows creation of an
empty blog post ready for editing. This would be a fairly trivial task
except that it also makes it convenient to include images. These are
specified by URL and the script downloads and resizes them for use in
the blog.

This commit also renames all existing post files to use hyphens rather
than underscores because this is the format that `create-post.py` will
use going forward. I decided to switch because I think hyphens are more
readable.