Support non-HTML content pages

[aprescott]

For now, the only non-HTML content allowed is Markdown in _posts and _drafts. This means that if you want an About page, it has to go in about.html and must be regular HTML.

Both of these should be allowed in the same site: foo.html and bar.md should work as expected.

It might be worth using Tilt for this.

It's possible that both this and #5 would be solved with the same thing. .html files could be defined to be "pass-through" by default, and .md files would use whichever renderer is used for Markdown files, then all files in _posts could have an implicit .md extension, otherwise you could have a different extension altogether, such as:

_posts
├── 2012-11-28-foo.html
├── 2012-11-29-bar
└── 2012-11-30-baz.md

[aprescott]

It's not quite clear how Liquid would play with this if there is a two-stage conversion process going on.