Feature request: Add option to show elapsed time since start

[Babalion]

Sometimes one wants the progress bar to show how long the task is already running.
Therefore, I suggest adding an option like showspeed=true but here showelapsed=true
which also shows the time since the loop started.

I guess the implementation is easy and straightforward.

Would love to see this in the future!

[jaredjeya]

This would be really useful since the ETA estimate is not always that reliable, since it assumes the time between updates is representative of the time for each iteration. It's especially bad for threaded/distributed tasks where you might expect a bunch of updates to come in at once, on a timescale much shorter than the actual time per task.

I wonder how tricky this would be to implement in practice, could be a fairly simple pull request.

[lgoettgens]

I just created a PR implementing this. Please have a look if this is what you imagined.

#356