twelve-days: default sing method

Add configuration defaults for nextercism

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.

If you haven't already, now would be a good time to do the following:

* add a rough estimate of difficulty to each exercise (scale: 1-10)
* add topics to each exercise
* choose *at most 20 exercises* to be core exercises (set core: true, and delete the unlocked_by key)
* for each exercise that is not core, decide which exercise is the prerequisite (max 1)

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.

rotational-cipher: move to end of track

rotational-cipher add to track

Removed the comma and set the difficulty to 1. :)

Put the slug at the end of the file

Updated the difficulty of the slug

Optimize CI Tests

Generate exercise READMEs from templates

We are working towards making exercises stand-alone—that is to say:
no more generating READMEs on the fly.

This will give maintainers more control over each individual exercise
README.

The template uses the Go text/template package, and the default templates
generate the exact same READMEs as we have been generating on the fly.

Move script parity comment into scripts README

Support a subcommand on configlet in CI

We will be adding subcommands to configlet.

For now, we've released a version of configlet which handles both the
old command:

    configlet path/to/track

as well as the new command:

    configlet lint path/to/track

This will let us update all the travis files to include the
subcommand before we release the version of configlet that requires
the subcommand.

Fix typo: 4 -> 5

Confirmed with canonical data: https://github.com/exercism/problem-specifications/blob/master/exercises/sum-of-multiples/canonical-data.json#L6
that this test should be 3 and 5

circular-buffer: add to track

Update to Gradle 4.0.1

https://twitter.com/gradle/status/883366530980864005

Update policy around language constructs

list-ops: update to match Kotlin track

Run unit test and journey test on seperate nodes
Gradle uses --configure-on-demand and --parallel where it wasn't present
Bundle install uses -j4

Removed starter implementations for exercises 21 and beyond, according to POLICIES.md.

bowling: add to track

poker: sort settings.gradle and tidy up

Fix formatting and Intellij warnings. Remove starter implementation as
this is after the first 20 exercises. Move poker in settings.gradle so
that it's sorted alphabetically.

sieve: create and validate starter implementation

twelve-days: create and validate starter implementation