Add MixedCase exercise names for README generation

Some languages use MixedCase for file names. This means that we will be able to make the template generate the actual command to run the tests for that particular exercise, rather than a generic example command using Foo or Hello World.

$ groovy ./{{ .Spec.MixedCaseName }}Spec.groovy

Make lint command more powerful

This adds two pieces of behavior to the lint command:

• verify that each exercise has a test suite, by checking all the file paths in the exercise subtree against the test_pattern in config.json. If the test pattern is missing, it defaults to the case-insensitive substring /test/i.
• verify that the maintainers config file config/maintainers.json is valid JSON

Add subcommand: uuid

This adds a new subcommand which will generate a fresh UUID to be used for an exercise in a track.

E.g.

$ configlet uuid
699d4aee-005e-4b80-f8f6-62af39c23c85952a7d5

For more details about the config.json structure/format, see https://github.com/exercism/docs/blob/master/language-tracks/configuration/README.md

Implement subcommands: lint, generate

As of this release, there is no longer support for the command configlet . or configlet path/to/track. If you want the behavior that was previously accessible via the top-level command, you must now use the lint subcommand explicitly:

configlet lint path/to/track

In addition to the lint command which verifies the track configuration, there is now a second subcommand, generate, which generates READMEs for the exercises in the track.

configlet generate path/to/track [--only $SLUG]

See the documentation for details.

Handle placeholder subcommand

We're going to be adding subcommands. This release allows configlet to work either with or without a subcommand.

Make error messages more expressive

This does not change the behavior in any way. It tweaks the error messages to provide a bit more guidance on what has gone wrong.

Remove support for deprecated track configurations

We no longer support:

• a "problems" key in the config (this is replaced by the "exercises" key)
• exercise implementations that live in the root directory (all exercises much be contained within the "exercises" directory)

We are no longer checking the "ignored" key in the config.

Support configurable solution paths

This adds support for an optional keysolution_pattern in the track's config.json file. The value is a string which is used to create a regular expression.

If the key is not present the default value of [Ee]xample will be used.

Deprecate "problems" key

This uses the "exercises" key to define the configured problems, falling back to "problems" if "exercises" is missing.

Patch release - ignore root img directory

Ignore root img directory

We're putting track icons in each of the language track repositories, in 'img/icon.png'. Ignoring the img dir by default lets us avoid putting each of those img dirs in the config.json under the "ignore" key.