Support progress without known total

I'd like to use a progress indicator showing time and thoughput, current total bytes.

Currently the counter box always shows the total:
```rust
// counter box
if self.show_counter {
let (c, t) = (self.current as f64, self.total as f64);
prefix = prefix +
&match self.units {
Units::Default => format!("{} / {} ", c, t),
Units::Bytes => format!("{} / {} ", kb_fmt!(c), kb_fmt!(t)),
};
}
```

Maybe a `show_total` could be added which switches to another counter format string like `{}`