Allow layout to be disabled

[mojavelinux]

Allow the layout to be disabled. In this case, Asciidoctor's header_footer option is enabled so that the page is converted to a standalone HTML document.

The layout would be disabled by unsetting the page-layout attribute in the document header:

:page-layout!:

See the Middleman AsciiDoc extension for an example.

[mojavelinux]

I've decided instead that explicit values will be easier to parse:

• false :: don't use a layout; create a standalone document
• ~ or null :: don't use a layout; don't create a standalone document
• empty or not specified :: use the default (inherited) layout
• otherwise, use the value to resolve a layout

[mojavelinux]

The following can be used to create a standalone document:

:page-layout: false

[mojavelinux]

In order for this to work, empty page attribute values need to be converter to an empty string instead of null. For example, the following should resolve to empty string:

:page-layout:

Currently, it's resolving to nil.

See #70.

[mojavelinux]

I think we leave room to support unsetting the page-layout attribute in the future.

:!page-layout:

We just won't do it yet (due to how this plugin works).

[mojavelinux]

I decided to implement the following rules:

• :page-layout: page — use the layout named page (e.g., page.html)
• not specified, :page-layout: or :page-layout: _auto — use the default layout (i.e., default for pages, post for posts)
• :!page-layout: or :page-layout: false — don’t use a layout; instead, generate a standalone HTML document
• :page-layout: ~ — don’t use a layout (often results in an incomplete HTML file)