Add option to exclude seconds from the summary output

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.

Start with the `summary` command and compare its current output with the examples in `doc/report.summary.txt`. Trace where the summary formats interval start, end, and duration values, then identify how configuration options are defined. Done means an option controls hours-and-minutes output, with the documented summary layout and minute rounding behaving as requested.

Written by the indexing model from the issue text.