v0.1.0 of feriados_pt

Portuguese national public holidays for Dart and Flutter. Calculates mandatory holidays for any year, including Easter-based moveable feasts, with correct handling of the 2013–2015 suppression period. Changelog excerpt: - Initial release. - getHolidays(year)— returns all mandatory Portuguese national holidays sorted by date. - isHoliday(date)— checks whether a date is a public holiday. - holidayAt(date)— returns the Holidayfor a date, or null. - Correct handling of the 2013–2015 suppression period (Lei 23/2012 / Lei 8/2016). - Easter calculated with the Meeus/Jones/Butcher algorithm.