1. "iso" and "xs" format strings now can be continued with flags like h, m, s and ms for setting shown accuracy, and nz, fz, u for time zone related behavior. For example, datetime_format="iso m nz". Tests and documentation is still lacking.

2. Generalized the date/time/date-time formatting machinery further, working towards user-provided formatter classes.
3. Started to make order among the date/time format tests