Soft wrap by Markdown only

I found an acceptable solution to do this:

1. Go to "Settings" -> "Keymap".
2. Look for "Main menu" -> "View" -> "Active Editor".
3. Assign a keyboard shortcup to "Use Soft Wraps" (for instance: Ctrl + Alt + w).

Now when you are editing a file, use this shortcut and it will enable the soft wraps only for this file.

It's 2019, and users of JetBrains apps now have automatic soft-wraps for Markdown documents - rejoice! 🎉🎊🥳

Alas it is not enabled by default, so you'll need to visit your preferences and check a box: On WebStorm 2019.2 at least, navigate to Editor > General > Soft Wraps and click your way to Markdown-soft-wrap bliss.