Added code and some error handling for retrieving the exercise file name from either config.json or via the passed-in slug name.
Changed the SLUG_RE code to accept an underscore as part of a valid slug name
Decided to not change the code to accept more than one exercise file since we don't currently have any mulit-file exercises in either the concept nor practice categories.
config.jsonor via the passed-in slug name.Decided to not change the code to accept more than one exercise file since we don't currently have any mulit-file exercises in either the concept nor practice categories.