Templates for chef prepare post and page

I use a number of custom properties in the front matter of my posts, and I wanted to have these properties automatically available from `chef prepare post`.

I'm using the templating engine here so that you can create custom twig templates for your page and post defaults. You can override the stock page and post templates by placing your own templates in `_content/prepare/post.html` or `_content/prepare/page.html`.

I also use all the extensions defined in auto_formats, so if you create ex. `_content/prepare/post.md` then `chef prepare post test-post` will create `path/to/test-post.md`.