nicer display formats for duration UDAs

When using an UDA with a duration (e.g. for the estimated time for completion), it seems that it can only be displayed in the ISO duration format. For example:

```
# .taskrc
uda.eta.type=duration
uda.eta.label=ETA
# (also add this to a report of your choice)
```

Then Taskwarrior only lists `default` and `indicator` as supported formats via `task colums eta`. I can set an ETA with e.g. `eta:5min`, but by default it gets displayed as `PT15M`. For dates, or rather for displaying the duration between now and a given date, Timewarrior supports other formats as well. These would highly increase the usability of duration UDAs.

So I think following format specifications could be added to durations (using the example from above):
- `age` for `15min`, useful when exact display is not required
- `countdown` for `00:15:00`, useful for second-precision output with hours as largest divisor
- `iso` for the current default `PT15M`