Decide whether the exercises in config.json should follow any specific order

In Exercism v1, we had the rule to organise all exercises in config.json by difficulty. In Exercism v2, this does not make much sense anymore since it doesn't reflect the order that students will get the exercises. So, what do we want to do?

First, decide whether core exercises should be sorted relative to side exercises:

* All core exercises go before any side exercise
* Core exercises interspersed with side exercises, according to whatever sorting rule is decided below:

Then, decide what is the sorting rule for side exercises:

* Continue to sort by difficulty even though it doesn't make sense
* Sort by (unlocking core exercise, difficulty)
* Explicitly no order; Wild West, anything goes.
* Some other idea goes here

If we decide on an ordering other than the Wild West option, consider making a Travis CI check that ensures that the order has been followed.