Add spacing to nested heading elements

[jocmp]

Link #211

Adds a custom mercury parser and subheading styling for factorio.com's blog. The root cause is that the site nests an <author> tag within an h3 heading tag.

Relates to

• https://github.com/jocmp/article_forge/commit/8b228057ff217901525b0a7c79dacf92cf210502