Add epiweek for interval option

[aspina7]

Hey,

Speaking to people at WHO on the course and the standard epiweek from CDC starts on a sunday. It would be good if interval = "epiweek" could be added as one of the options for interval where it defaults to a week starting on a sunday.

Possibly as well it would be nice to be able to put in Mon, Tues, Wed, Thurs ... to define the start day of the week.

I know you can put the start_date to solve these problems but I think this would be easier for the end user.

[caijun]

Also see the discussions in #24 and #55

The US CDC uses MMWRweek.

Currently interval = week refers to ISO week, the first day of which is Monday. It should be easy to support MMWR week. The week values for interval can be "ISOweek" and "MMWRweek", or other week formats that would be supported in the future.