add option to display calendar week

[justb3a]

Adds an option which enables a column containing week numbers.
(@see: #66 #192)

Extended prop

Prop	Type	Default	Description
calendarweek	Boolean	false	To display the calendar week

[coveralls]

Coverage remained the same at 100.0% when pulling 41cdbb9 on justb3a:feature/week-numbers into bf28bca on charliekassel:master.