[blog] Posts should be directories

[preaction]

If the post is a directory, we get quite a few awesome things:

1. Images can go in the same directory for easy deployment and linking (fixes [blog] static files inside store do not get deployed #228)
2. The .html extension goes away (fixes [blog] index.html should be implied everywhere #165)
3. We can have multiple-page posts in the future
4. We could add additional comment documents in the directory in the future

Since this breaks a few things, we should do this right away

[preaction]

This is a breaking feature because the existing markdown pages will not have a directory. We need to support the old way of doing things for existing sites, or perhaps just upgrade the existing posts into directories.

[preaction]

I like the idea of supporting the old way, warning about it, providing a small shell script to fix it, and removing it completely at 1.00.