implement strftime equivalent for undate objects

[rlskoeser]

roughly equivalent to datetime.strftime but aware of the partially known, unknown, uncertain, etc.

may be able to adapt from related work from Shakespeare and Company Project, which has a custom django template tag for partial dates that works like the django built-in date template tag with handling for unknown parts of the date

• preliminary version: support available formatters in the undate package: implementation in preliminary str parse and format methods #90
• basic version: support syntax analogous to datetime strftime subset for dates - adapt S&co template code if possible
• expanded version: support different locales (maybe using babel ?)