## join
### Instructions
Write a function that returns the concatenation of all the `string` s of a slice of `string` s **separated** by the separator passed in the argument `sep` .
### Expected function
```go
func Join(strs []string, sep string) string {
}
```
### Usage
Here is a possible program to test your function :
```go
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 )
