Parse the text value in code blocks

[stinoga]

This resolves an issue where code block child nodes are returning
strings only, rather than a TextNode. This causes them to fail to
render when imported.

[sstur]

Hey @stinoga, thanks for your contribution!

Can you add a test case to this file, so I can better understand what issue this solves?

https://github.com/sstur/draft-js-utils/blob/master/packages/draft-js-import-markdown/src/__tests__/stateFromMarkdown-test.js

Ideally some piece of markdown that is incorrectly parsing with the current implementation and fixed with your PR.

Thanks in advance.