[feature request] adapt to print with given digit

```rust
let m = 10000.0 / 3.0;
// print 3333.33
println!("{:.2}", m);
// print 3,333.3333333333335
println!("{:}", m.separated_string());
// expectedd print 3,333.33
// actual print 3,
println!("{:.2}", m.separated_string());
```

rustc version: 1.52.1
