Control layouts through front matter

[hmans]

We need to have a way to control which layout files are applied to a node through setting a value in its front matter. The following use cases should all be possible:

• rendering a page with all layouts applied normally (the default case, currently implemented)
• rendering a page with no layouts at all
• rendering a page with one or more specific layouts (specified by node selector?)
• rendering a page with one or more specific layouts, plus any layouts encountered higher up the tree

Applying no layout at all:

layout: []

or

layout: false

Applying a specific layout:

layout: "/_post"

Applying multiple layouts:

layout: ["/_post", "/_container"]

[hmans]

Pull request: #24