Display download's elapsed time as seconds

[ducaale]

Since we're planning to support HTTPie's --meta flag for displaying the response's elapsed time, I thought we should simplify how we display elapsed time everywhere.

Before:

Downloading 10.00MiB to "<stdout>"
Done. 10.00MiB in 4s 810ms 951us 800ns (2.08MiB/s)

After:

Downloading 10.00MiB to "<stdout>"
Done. 10.00MiB in 10.892807s (940.07KiB/s)

[ducaale]

I could have used as_secs_f64() here, but the resulting output would be too lengthy.

[blyxxyz]

To give the output the right length you can format it with {:.5} instead of {}. HTTPie does the same.