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

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

Move contributing info into CONTRIBUTING file and add mention of rebases

spiral-matrix: add to track

Based on v1.0.0 of the canonical data:

https://github.com/exercism/problem-specifications/tree/28a32203b96ebaecac6a6968831bd66718d8ba30/exercises/list-ops

Plus modifications that are being discussed in

https://github.com/exercism/problem-specifications/issues/826

complex-numbers: add to track

Standardize order of test annotations

Create RESOURCES.md and add resources

Clean up exception-related hints

Formatted Stack Overflow with a space in between.
Capitalized Java in the Reddit link and removed the claim that it was official.
Removed Gitter room link
The documentation is official