fix rendering of markdown in unions

[nextdude]

A tweak to add newlines around descriptions when rendering unions so the markdown is properly recognized downstream by markdown processors.

Fixes #71

[coveralls]

Coverage increased (+0.1%) to 78.466% when pulling a9bfb2a on nextdude:fix-union-rendering-markdown into 7b6f45f on exogen:master.

[exogen]

Going to merge this, then add extra newlines around the description like we do for other multiline Markdown. I'm late on this but thank you!