Format block with label

For example,
```rust
'empty_block: {}

let result = 'block: {
    if foo() {
        break 'block 1;
    }
    if bar() {
        break 'block 2;
    }
    3
};
```
cc https://github.com/rust-lang/rfcs/blob/master/text/2046-label-break-value.md.