add an options to show download progress

[axetroy]

[rhysd]

Would be good idea. This is a library so directly calling Print in library code would not be useful. So I think It's nice to provide some callback in a field of Updater struct. User can determine what should be done on each tick of download by setting the callback function.

[rhysd]

Or providing io.Writerfield for writing progress bar would be another option.