[Time] Support for Fiscal Calendar

It seems that all of the date calculations are assuming the ISO 8601 calendar (at least for the EN language). It would be interesting to be able to use other calendars, such as the fiscal calendar used by companies in the US (see https://en.wikipedia.org/wiki/Fiscal_year and https://en.wikipedia.org/wiki/4–4–5_calendar).

I am using duckling in a project where such an issue has arisen, and the best solution would be to support it in duckling.

As I understand, the support for this may need first #29 to be complete.