Break YAML into a separate array

If we can break all the YAML header stuff into a separate array, it will be easier to conditionally `push` items into it.

For instance, if a post is a *Draft*, we may want to include the `published: "false"` header, but we don't need to include anything at all if the post is not a draft.
Merge pull request #4 from pathawks/patch-2

Break YAML into a separate array