Add builder-like with_finish method to ProgressBarIter.

[matthewhchan]

This lets you more concisely set the finish behavior of the progress bar when using ProgressBarIter.

For example, to show a progress bar and keep it displayed when it finishes, you can do

v.iter().progress().with_finish(ProgressFinish::AndLeave)

instead of

v.iter()
        .progress_with(ProgressBar::new(v.len() as u64).with_finish(ProgressFinish::AndLeave));

Closes #545

[djc]

Thanks!