Add support for additional page layout options

[mojavelinux]

Add the following permutations of page layout:

• _auto_layout - use the automatic layout; same as blank value
• false - don't apply a layout; create a standalone document
• ~ - don't apply a layout, but don't create a standalone document either

This change aligns the behavior of the Middleman extension with the Jekyll plugin. See https://github.com/asciidoctor/jekyll-asciidoc#specifying-a-layout