Extend single_match to handle return

In the following code:

``` rust
enum Foo {
    Bar(u8), Baz
}

fn main() {
    match Foo::Bar(3) {
        Foo::Bar(_) => { /* do something */ },
        _ => { return; }
    }
}
```

Clippy produces no warning, but it could produce the single_match warning instead.
