Add yamlWriter.R for hackers

Following a discussion with @jtleek about authoring swirl content, it seems that editing a text document directly will be preferred by many. I'd like to include this as an alternative to the swirlify authoring app. I've made a few small changes to the original, which was created by @WilCrofter.

I haven't done anything to make this conform to R package standards (i.e. exported functions, roxygen2 docs, etc.) More to come...