Support progress without known total

[lambdaupb]

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

Currently the counter box always shows the total:

        // 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 {}