Add support for partial holidays

Some countries have partial holidays, holidata should support it.

What "partialities" are there?
* `` - day is workday until `start`, holiday afterwards (e.g. Christmas Eve in Iceland is holiday from 13:00)
* `start - end` - holiday only between `start` and `end` (never say never, but is that possible...?)

Review the proposed partial-holiday forms in the issue, including ``, `start>`, and `start - end`, and determine which semantics and country examples should be supported. Done means holidata can represent and calculate the agreed partial-holiday cases, with clear handling for the unresolved examples.

Written by the indexing model from the issue text.