Add --time-colon option

The --time-colon option will display the value for a seconds
based commodity as real hours and minutes.

For example 8100 seconds by default will be displayed as 2.25
whereas with the --time-colon option they will be displayed
as 2:15.

I've made much use of the feature, much to my customers' delight,
for 7:45 hours makes much more sense than 7.75 in a billed timesheet.

I'm sure the introduced changes can be improved and I look forward
to your feedback