Allow initialisation from io.Reader

[rusq]

Problem

layout package expects the yaml file to be available on disk. With embedding, it could be convenient to store the yaml on an embedded filesystem, or embed as a string at compile-time.

Proposed Solution

• Implement FromReader and ReadDefault that allow to read the layout from any reader.
• Modify New and NewDefault to use FromReader and ReadDefault correspondingly.

This allows to use the embedded layout.yaml file, i.e.:

//go:embed layout.yaml
var layoutYAML string

func foo() {
    //...
    l, err := layout.FromReader(strings.NewReader(layoutYAML))
    if err != nil {
        // handle err
    }
    // use layout
    // ...
}

[rusq]

As I experiment with layout, I think it's better to add the support for fs.FS. I'll create a PR when I have something, experimenting with layout ATM. Would be interesting to know your thoughts @demget .