Add tooling for concept exercises

Currently I have a couple things in mind:
- [ ] a generate command for creating concept exercises
- [ ] ~~make the update command work with concept exercises (currently generate_exercise_files looks inside of exercises/practice)~~
- [ ] any helpful lints or checks I think of that configlet doesn't do (nothing specific so far)

Thought I'd open this issue first instead of just submitting a cold PR.