[v3] Implement new Concept Exercise: variadic-functions

This issue describes how to implement the `variadic-functions` concept exercise for the Clojure track.

## Getting started

**Please please please read the docs before starting.** Posting PRs without reading these docs will be a lot more frustrating for you during the review cycle, and exhaust Exercism's maintainers' time. So, before diving into the implementation, please read up on the following documents:

- [The features of v3](https://github.com/exercism/v3/blob/master/docs/concept-exercises.md).
- [Rationale for v3](https://github.com/exercism/v3/blob/master/docs/rationale-for-v3.md).
- [What are concept exercise and how they are structured?](https://github.com/exercism/v3/blob/master/docs/features-of-v3.md)

Please also watch the following video:

- [The Anatomy of a Concept Exercise](https://www.youtube.com/watch?v=gkbBqd7hPrA).

## Goal

The goal of this exercise is to teach the student how to use variadic functions in Clojure.

## Learning objectives

The student will learn:

- That functions may also define a variable number of parameters
- Usage of `&`
- Examples in `clojure.core` such as `str` and `format`

## Out of scope

- Multi-arity functions
- Creating named parameters through destructuring args

## Concepts

- `default-arguments`
- `evaluation`
- `functions`
- `rest-parameters`

## Prerequisites

- `basic-functions`

## Resources to refer to

### Hints

- [Clojure Guide: Functions](https://clojure.org/guides/learn/functions)

### After

- [clojure-doc.org - Functions in Clojure](http://clojure-doc.org/articles/language/functions.html#variadic-functions)

## Representer

This exercise does not require any modifications to the track's representer.

## Analyzer

This exercise does not require any modifications to the track's analyzer.

## Implementing

To implement a concept exercise, the following files must be created:

```
languages
└── clojure
└── exercises
└── concept
└──
├── .docs
| ├── instructions.md
| ├── introduction.md
| ├── hints.md
| └── after.md (optional)
├── .meta
| |── design.md
| └── Example.clj
├── .clj
└── _test.clj
```

## Help

If you have any questions while implementing the exercise, please post the questions as comments in this issue.