Add "week of year" expression

[isovector]

My current contract for work is to do X hours of work a week. I use arbtt for everything else, and would like to have a convenient means of checking how much work I've done this calendar week.

This PR adds support for it.

[isovector]

I've added the following to my config:

week of year $now == week of year $date ==> tag this-week,

and it is working as expected :)

[isovector]

I've updated the documentation with the new expression!