Allow custom layouts to be set in page meta. From the @pomaxa fork.

### Usage:

/*
Title: I Love Kittens
Author: James Doyle
Layout: post
*/

This will load the `post.html` file from your chosen theme. If the `Layout` key was absent it would just fallback to the `index.html` file.

Taken from @pomaxa's fork!

This feature is in a couple other forks and I believe it is strongly needed.