MINOR: cli: add an option to display the uptime in the CLI's prompt

MINOR: cli: add an option to display the uptime in the CLI's prompt

Entering "prompt timed" toggles reporting of the process' uptime in
the prompt, which will report days, hours, minutes and seconds since
it was started. As discussed with Tim in issue #2145, this can be
convenient to roughly estimate the time between two outputs, as well
as detecting that a process failed to be reloaded for example.