feat: option to keep showing elapsed time on finish

[hexnaught]

Add an option to keep the render of time elapsed when the bar has finished.

bar := progressbar.NewOptions64(
	10,
)
// Output:
//  100% |████████████████████████████████████████|         

bar := progressbar.NewOptions64(
	10,
	progressbar.OptionShowElapsedTimeOnFinish(),
)
// Output:
//  100% |████████████████████████████████████████|  [10s]

Resolves schollz/progressbar#128