time difference inflected as before/after

I'd like to see a feature like the following:

```
>>> p.date_diff(datetime.timedelta(minutes=-30))
'00:30:00 before'
>>> p.date_diff(5)
'00:00:05 after'
>>> p.date_diff(0)
'at'
```

I would expect one to use the result to produce nice output for a time difference like:

> Your flight departed 00:00:05 after its scheduled departure.

It would be nice if it accepted a time formatter such that it could alternately emit:

```
>>> p.date_diff(datetime.timedelta(minutes=-30), fmt=nice_time)
'30 minutes before'
>>> p.date_diff(5, fmt=nice_time)
'five seconds after'
>>> p.date_diff(0, fmt=nice_time)
'at'
```

Perhaps that inflection is too trivial for a library like inflect. Or perhaps there are nuances I haven't yet considered.

The issue proposes a new p.date_diff entry point with before/after and at outputs, plus an optional fmt callable. No files or tests are named, so begin by inspecting the existing inflect API and test layout; done means the shown positive, negative, zero, and custom-format examples behave as specified.

Written by the indexing model from the issue text.