Proper Sundays

[djjudas21]

With the creation of Ordinary Time in #13, we should probably add support for Proper Sundays which are explained here.

At the moment, this library fabricates the names of Sundays by appending the week number to the period time, e.g. Epiphany 3. Ordinary Time is not numbered like this and instead should refer to Proper Sundays, which are tied to calendar dates (and therefore to the Christmas cycle). Not all of the earlier Proper Sundays will be used, depending on where Easter falls.

Currently, the library would come up with things like Ordinary Time 3 but it also needs to have a way of calculating the Proper Sundays and referencing them during Ordinary Time.