New feature: print runtime of command in human readable format in prompt

[sinexton]

It would be nice to see the total runtime of the last command in human readable format in the prompt if it took more then a couple of seconds to run.

If someone runs time heavy tasks then it may improve efficiency and it can be automatic and stay hidden if no heavy tasks have been run. An implementation for converting seconds to human format:

def time_human( sec )
	res = nil
	mul = 1.0
	[ [60,"s"], [60,"m"], [24,"h"], [30.4375,"d"], [12,"mo"], [-1,"y"] ].each{|time,text|
		mul *= time
		if sec < mul or time < 0
			res = "#{ round( sec.to_f * time / mul ) }#{ text }"
			break
		end
	}
	return res
end