Add option to exclude seconds from the summary output

[joelostblom]

Currently, the output of the summary command shows the number of seconds for each interval, like so:

Wk  Date       Day Tags               Start    End       Time    Sub
--- ---------- --- ------------------ -----    -----     -----   -----
W17 2016-04-25 Mon Holiday
W17 2016-04-26 Tue Research, B23       9:00:00 12:30:12 3:30:00
                   Research, B23      13:30:00 18:00:13 4:30:00  8:00
W17 2016-04-27 Wed Training Security   9:00:00 12:00:42 3:00:00
                   Training Privacy   13:00:00 17:00:14 4:00:00  7:00
W17 2016-04-28 Thu Staff Meeting       8:45:00 10:30:13 1:45:00
                   Research, A44      10:40:00 12:00:12 1:20:00
                   Research, A44      12:45:00 17:52:12 5:07:00  8:12
W17 2016-04-29 Fri Research, C17       8:41:00 12:00:22 3:19:00
                   Dept Meeting       13:00:00 14:30:12 1:30:00
                   Planning           14:30:00 15:30:13 1:00:00
                   Research, C17      15:30:00 17:40:12 2:10:00  7:59
--- ---------- --- ------------------ ----- ----- ----- -----
                   Total                                          31:11

I find this difficult to read as there are so many numbers close to each other, so I would like to remove the seconds and only show hours and minutes (ideally rounded to the nearest minute) as shown in the examples in this file https://github.com/GothenburgBitFactory/timewarrior/blob/develop/doc/report.summary.txt#L26:

Wk  Date       Day Tags               Start End   Time  Sub 
 --- ---------- --- ------------------ ----- ----- ----- ----- 
 W17 2016-04-25 Mon Holiday 
 W17 2016-04-26 Tue Research, B23       9:00 12:30 3:30 
                    Research, B23      13:30 18:00 4:30  8:00 
 W17 2016-04-27 Wed Training Security   9:00 12:00 3:00 
                    Training Privacy   13:00 17:00 4:00  7:00 
 W17 2016-04-28 Thu Staff Meeting       8:45 10:30 1:45 
                    Research, A44      10:40 12:00 1:20 
                    Research, A44      12:45 17:52 5:07  8:12 
 W17 2016-04-29 Fri Research, C17       8:41 12:00 3:19 
                    Dept Meeting       13:00 14:30 1:30 
                    Planning           14:30 15:30 1:00 
                    Research, C17      15:30 17:40 2:10  7:59 
 --- ---------- --- ------------------ ----- ----- ----- ----- 
                    Total                                31:11

It would be helpful if a configuration option could be added for this.