There is a discrepancy in how control characters (specifically   `\n`) are handled when calling `.width()` on a `char` versus a `&str`.

In version 0.2.2 :

```rust
use unicode_width::{UnicodeWidthChar, UnicodeWidthStr};

fn main() {
    let c = '\n';
    let s = "\n";

    println!("char width: {:?}", c.width()); // Returns None
    println!("str width:  {:?}", s.width()); // Returns 1
}

Is that a bug or a feature?
