Implement new Concept Exercise: time-duration

This issue describes how to implement the `time-duration` concept exercise for the Go track.

## Getting started

The docs can be found here
- [What are Concept Exercises?](https://exercism.org/docs/building/product/concept-exercises)
- [Concept Exercises](https://exercism.org/docs/building/tracks/concept-exercises)
- [Concepts](https://exercism.org/docs/building/tracks/concepts)

## Goal

The goal of this exercise is to teach the student how concept time and duration is implemented in Go.

## Learning objectives

- Define the relation between time.Time and time.Duration.
- Know how to measure time using time.Since ( e.g. execution time of a function -- no `defer` !) .
- Know how to use `time.Add` and `time.Sub`
- Know about time units and maybe do some calculations

## Out of scope

- Measuring time should to include `defer`
- Sleep, After, ticker etc.

## Concepts

- `time-duration`

## Prerequisites

- `time`

## Resources to refer to

- [Time package](https://golang.org/pkg/time/)

## Story Idea

There is already a possible story including tasks in the story list: https://exercism.org/docs/building/tracks/stories/datetimes-time-keeper
This might be helpful as a template.

## Help

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