Implement subset of strftime/strptime POSIX standard

This implements a subset of the Python datetime `strftime`/`strptime`
functionality. Some tokens have not been implemented due to
conflict with ISO 8601 interpretations (e.g. week day (`%w`), where
the strftime (US-derived) week starts on Sunday) or due to
locale/language dependencies.

@matthewrmshin, please review.