Pre-process drafts to publish based on metadata headers

When the site is generated, there should be a pre-processing step that takes any file in `_drafts` that has an appropriate header and publishes it using the current timestamp.

For example, if `_drafts/foo` has the content:

```
title: Testing
publish: now

A post that will be published.
```

Then before the actual HTML generation step, `_drafts/foo` should be moved into `_posts/<CURRENT_DATE>-foo` with `Created: <FULL_TIMESTAMP>` in the headers.