Add methods that are related with substitute holiday

Add methods to Holiday interface that is related with substitute holiday.

Original

If holiday is substitute holiday, Original method returns original holiday.

ex:

d, _ := flagday.PublicHolidayOf(2009, 5, 6)
fmt.Println(d.Kind() == flagday.SubstituteHoliday) // true
fmt.Println(d.Original().Time().Format("2006/1/2") // 2009/5/3

IsSubstituted

If holiday is original holiday, IsSubstituted method returns true.

ex:

d, _ := flagday.PublicHolidayOf(2009, 5, 3)
fmt.Println(d.IsSubstituted()) // true

First release

Merge pull request #2 from pinzolo/gomodule

Add go.mod