## join
### Instructions
Write a function that simulates the behaviour of the `Join` function in Go. This function returns the concatenation of all the strings of a slice of strings **separated** by a separator passed in argument.
### Expected function
```go
func Join(strs []string, sep string) string {
}
```
### Usage
Here is a possible program to test your function :
package main
import (
"fmt"
"piscine"
)
func main() {
toConcat := []string{"Hello!", " How", " are", " you?"}
fmt.Println(piscine.Join(toConcat, ":"))
And its output :
```console
$ go run .
Hello!: How: are: you?
$
### Notions
- [strings/Join](https://golang.org/pkg/strings/#Join)
