## rotate

### Instructions

In this exercise, if you do not know about it already, you will learn about the rotational cipher "ROT13".

A ROT13 applied the Latin alphabet:
```
- Plain:  abcdefghijklmnopqrstuvwxyz
          ||||||||||||||||||||||||||
- Cipher: nopqrstuvwxyzabcdefghijklm
```

You will create a similar `rotate` function that is a better version of the ROT13 cipher.

Your function will receive a `String` and an `i8`, and will rotate each letter of that string by the number of times described by the second argument. Positive numbers rotate to the right, negative numbers rotate to the left.

Only letters should be rotated. Numbers and punctuation should be left unchanged.

### Expected functions

```rust
pub fn rotate(input: &str, key: i8) -> String {

}
```

### Usage

Here is a program to test your function.

```rust
use rot::*;

fn main() {

    println!("The letter \"a\" becomes: {}", rotate("a", 26));
    println!("The letter \"m\" becomes: {}", rotate("m", 0));
    println!("The letter \"m\" becomes: {}", rotate("m", 13));
    println!("The letter \"a\" becomes: {}", rotate("a", 15));
    println!("The word \"MISS\" becomes: {}", rotate("MISS", 5));
    println!(
        "The decoded message is: {}",
        rotate("Gur svir obkvat jvmneqf whzc dhvpxyl.", 13)
    );
    println!(
        "The decoded message is: {}",
        rotate("Mtb vznhpqd ifky ozrunsl ejgwfx ajc", 5)
    );
    println!(
        "Your cypher wil be: {}",
        rotate("Testing with numbers 1 2 3", 4)
    );
    println!("Your cypher wil be: {}", rotate("Testing", -14));
    println!("The letter \"a\" becomes: {}", rotate("a", -1));
}
```

And its output:

```console
$ cargo run
The letter "a" becomes: a
The letter "m" becomes: m
The letter "m" becomes: z
The letter "a" becomes: p
The word "MISS" becomes: RNXX
The decoded message is: The five boxing wizards jump quickly.
The decoded message is: Ryg aesmuvi nkpd tewzsxq jolbkc foh
Your cypher wil be: Xiwxmrk amxl ryqfivw 1 2 3
Your cypher wil be: Fqefuzs
The letter "a" becomes: z
$
```

### Notions

- [patterns](https://doc.rust-lang.org/book/ch18-00-patterns.html)