Add elapsed time to progress bar

[devonhollowood]

I think it's pretty reasonable to have this display by default. If you want to hide it behind an option, though, or you want me to change the format / wording, please let me know!

Fixes #416.

[sharkdp]

Thank you for your contribution!

Hm. I think the idea of the ticket was to only show this for the very first run. I'm not sure about the added value if we show this always. I think less information might be the better default here. Maybe we could simply replace the "Initial time measurement" message with: "Initial time measurement: {elapsed}"? would that work?

[devonhollowood]

Maybe we could simply replace the "Initial time measurement" message with: "Initial time measurement: {elapsed}"? would that work?

This doesn't work, because template arguments in the ProgressBar's message don't get substituted. So we need a slightly deeper change. I'll see what I can come up with.

[devonhollowood]

Okay, I think this should work. Here's a gif of what it looks like on my machine.

[cblair]

Still something that would be useful, I was suprised hyperfine didn't have this already.