Add configuration defaults for nextercism (#576)

We will be moving to a tree-shaped track rather than a linear one, as described in the [progression & learning in Exercism](https://github.com/exercism/docs/blob/master/about/conception/progression.md) design document.

In order to support this, we need to expand the metadata that exercises are configured with.

Note that 'core' exercises are never unlocked by any other exercises. Core exercises appear in the track in the order that they are listed in the array.

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.

Until now we have operated with a separate deprecated array. These are now being moved into the exercises array with a deprecated field.

With these defaults the track in nextercism will have no core exercises, and all the exercises will be available as 'extras' from the start.