generate-challenge-json.js bug with incomplete chapters

[Ethan-Arrowood]

From: #23

Discovered bug in generate-challenge-json.js. When a new challenge is added to an empty chapter, and the new challenge is not the first challenge in the chapter, it will be added as if it is the 'first'. When generate-challenge-json.js is ran again it will re-add the (not-first) challenge to the chapter producing unpredictable chapter structure. Will open a new issue and begin work on this soon.

I'll figure it out. Might need to add some sort of 'skip' functionality to renderMap, and loadChallenges for 'empty challenges'?