Add progression through exercises

[SaschaMann]

See exercism/discussions#175.

First attempt at a progression order through the track. Looks like this:

[jonmcalder]

Cool! FYI, my understanding is that (for now at least) only core exercises should unlock other exercises.

e.g. from exercism/discussions#75:

Aside from those initial optional exercises, all other optional exercises (core: false) should be unlocked_by the slug of a (a single) core exercise.

You've got sequential unlocking in nucleotide-count -> scrabble-score -> etl

[SaschaMann]

@kytrinyx' PR #73 mentioned the following:

Non-core exercises depend on only one exercise (unlocked_by: ). At the moment we are assuming that this is a core exercise, but there is no reason that it needs to be.

so I thought such progression would also be fine. Javascript also uses it according to the visualisation example.

---

Added some non-core, non-bonus exercises so we fill the following criteria:

If possible, leave 3 or 4 simple exercises as (core: false, unlocked_by: null), as this will provide new participants with some exercises that they can tackle even if they have not finished the first core exercise.

[jonmcalder]

Ah ok cool. I must have overlooked that somehow! My apologies.

[kytrinyx]

I'm pretty sure what you have here will work. Once we get the prototype launched we'll be able to test it out and tweak it if not.