Feature: access to stats after `Done()`

[brendensoares]

I'd like to show a summary status after the progress is completed.

Would be nice to render a final bar format after completion. e.g. during:

(█████               ) 25.0% 3.0 ops/s

and after:

(████████████████████) 100%

It may be better to simply allow the calling code to access the bar instance's stats props to display its own summary. e.g.

Processed 5,300 units in 18s (avg 9 units/sec)

Summary of proposed features:

1. Add optional WithFinalFormat() e.g. :bar :percent to replace bar's in-progress, initial format
2. Export Bar stats props like rate, startedAt, and (assuming the duration token gets added) duration