Fix markups logic

> https://medium.com/@ericclemmons/jest-snapshots-for-storybook-5bf36b5e5a3a

Complex nesting of markups is hard to parse.

This warrants a unit test IMO.

https://github.com/jaysalvat/medium-json-parser/blob/master/src/MediumJsonParser/Parser.php is a good example for this, but I'm not sure it's flawless.

Start by reviewing the markup parsing behavior in the project and the referenced MediumJsonParser/Parser.php example. Add a unit test covering complex nested markups, then confirm the parser handles that case correctly and produces the expected Markdown output.

Written by the indexing model from the issue text.