Add function to pretty-print durations

Submitted by Bastien Nocera `@hadess`

We would also want to be able to force showing the hour item, if the corresponding item has an hour field.

eg., progress as I've just started watching a film:
00:00:15/1:25:00

And usefully, print time left:
00:00:15 [===============] -1:24:45

API bikeshedding appreciated.