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 `{}`

The issue points to the counter box logic in the codebase, likely in a file like `src/lib.rs` or `src/progress.rs`. Look for the `show_counter` field and the `Units` enum. Add a `show_total` boolean field to the progress bar struct and adjust the formatting logic to conditionally show the total. Check existing tests for the progress bar to understand how to verify the change.

Scritto dal modello di indicizzazione a partire dal testo della issue.