feat: register fifteenth day

internal/2015/register.go

@@ -3,6 +3,7 @@ package year2015
 import (
 	_ "advent-of-code/internal/2015/DayEight"
 	_ "advent-of-code/internal/2015/DayEleven"
+	_ "advent-of-code/internal/2015/DayFifteen"
 	_ "advent-of-code/internal/2015/DayFive"
 	_ "advent-of-code/internal/2015/DayFour"
 	_ "advent-of-code/internal/2015/DayFourteen"