Support YAML front matter

[weierophinney]

The github specification now allows/defines YAML front matter for mustache documents; this provides a way to create default values for the view.

In all cases, front matter begins and ends with a line containing only ---. You can have a single front matter section, or multiple front matter sections.

A single front matter section:

---

names: [ {name: chris}, {name: mark}, {name: scott} ]

{{#names}}
Hi {{name}}!
{{/names}}
Multiple front matter sections:

---

name: chris

name: mark

name: scott

Hi {{name}}!
Technically, this is a feature of the mustache command, and not necessarily required by implementations. However, when considering things like template inheritance, it could be a useful feature.

[weierophinney]

This package looks promising for parsing the frontmatter: https://github.com/mnapoli/FrontYAML

I'm not sure if this should be part of Mustache itself, or part of a facade class. It touches base on several aspects:

• Lexing. It would need to execute prior to lexing, keeping the parsed YAML front matter as a view to associate with the given template, and passing the remainder of the document on to the lexer.
• Rendering. It would need to pass the parsed YAML front matter on as a view against which to merge a user-supplied view when doing substitutions.