[v3] Add key features and tags

[bobbicodes]

Issues: #316 and #317.

[ErikSchierboom]

As it looks like nobody else is reviewing, LGTM!

[ErikSchierboom]

@porkostomus There are a couple of issues with the tags and key features according to configlet (must have been a very old version that checked the PR :)):

The value of `key_features.content` that starts with `Clojure extends Lisp's co`... is 139 characters, but it must not exceed 100 characters:
/home/erik/exercism/clojure/config.json

The value of `key_features.content` that starts with `Clojure's primary program`... is 140 characters, but it must not exceed 100 characters:
/home/erik/exercism/clojure/config.json

The value of `key_features.content` that starts with `Clojure provides the tool`... is 166 characters, but it must not exceed 100 characters:
/home/erik/exercism/clojure/config.json

The value of `key_features.content` that starts with `Clojure is impure, in tha`... is 133 characters, but it must not exceed 100 characters:
/home/erik/exercism/clojure/config.json

The value of `key_features.content` that starts with `Clojure, being a practica`... is 224 characters, but it must not exceed 100 characters:
/home/erik/exercism/clojure/config.json

The value of `key_features.content` that starts with `Clojure is designed to be`... is 234 characters, but it must not exceed 100 characters:
/home/erik/exercism/clojure/config.json

The `tags` array contains `used_for/used_for/frontends`, which is not one of the allowed values:
/home/erik/exercism/clojure/config.json

But those can be fixed in a follow-up PR :)