Refer learners to exercism.io or other practice resources after lesson?

Hello :-) This is a repost of [carpentries/mentoring#31](https://github.com/carpentries/mentoring/issues/31) where this repo was mentioned as a more fitting location.

How about mentioning some relevant resources like the [exercism.io language tracks](http://exercism.io/languages/) at the end of a lessons or in some extra page called "continue practicing", "where to go from here", "ingrain what you learned", "stick to it to become a pro", "explore more", or similar.

What do you think?

Tasks for me: send PR with...

- [ ] test in [`r-novice-inflammation`](https://github.com/swcarpentry/r-novice-inflammation/tree/lesson-example-163-exercism/)
- [ ] template site to this repo
- [x] define Jekyll variables for exercism-available languages
- [x] free-from part
- [ ] specific examples to lesson repos
- [ ] `if-not git` switch with list of available tracks to exclude