Add date formatting for datetime etc instances

```
It's useful to be able to specify a format for datetime values. I've attached a
diff that provides this functionality, basically by copying the float and int
formatting. It's applied to any object with a "strftime" method, which catches
date, time and datetime.

The validation of the format is pretty minimal, because you can put basically
anything in a strftime format. I wondered whether it was worth it, but I left
it as it's easier to extend the checks in the future if needed.
```

Original issue reported on code.google.com by `p.f.mo...@gmail.com` on 28 Aug 2014 at 1:31

Attachments:
- [prettytable.diff](https://storage.googleapis.com/google-code-attachments/prettytable/issue-57/comment-0/prettytable.diff)

Start by reviewing the attached prettytable.diff and the existing float and int formatting implementation. Confirm how the proposed formatting applies to objects with a strftime method, including date, time, and datetime instances, and ensure the requested formatting behavior is covered.

Written by the indexing model from the issue text.