Add support for week dates and ordinal dates

[movermeyer]

I've gotten tired of having to always say "ciso8601 doesn't support the entirety of the ISO 8601 spec, only a popular subset."

Investigate how much of a performance hit adding support for these would be.

I suspect now that the performance is dominated by the time it takes to initialize the datetime object, that we could add this without taking any noticeable hit.