Create an extension to generate a DocFX yaml content file

[Cloud-Awesome]

Admittedly, it's not markdown at all, but it's a very quick, simple and useful addition (c.f. the DocfxYamlHeader class) and fits well with the other DocFX functionality so may as well keep them all together!

Related to #15

e.g.

var landingPage = new DocfxYamlFile(someFolderLandingPage)
// where someFolderLandingPage is a POCO object, based on a JSON schema

[Cloud-Awesome]

A great test/use case/extension for this would be extending MdDocumentSet to accept an IEnumerable of classes and produce a set of markdown files, all following the same structure but with different data.