provide optional finish message

[yarikoptic]

Relates to #40 asking for the option to simply remove progress bar upon completion. I guess I could accomplish it if #40 was possible, by redoing entire line myself. But it might be nice if there was also possibility to specify the final "widgets" which are used whenever download is finished. Then it should be possible to go from

url: DOWNLOAD PROGRESS

to e.g.

url: downloaded_filename [download_time]

without manually "implementing" it (assuming #40 is addressed)

[willwhitney]

This would be a really strong addition.