Allow to configure week numbering system

[jotaen]

When doing klog report --aggregate week or klog total --last-week, it follows the ISO8601 definition of the week start:

• The first day of the week is Monday
• The first week of the year is the week that contains the first Thursday of the year

There are different practices than ISO, however:

• The first day of the week can also be Sunday (American)
• The week number always starts at 1st January, even is that would be only half a week then.

Without the ability to persist configuration, the easiest is probably to let users set an environment variable, like KLOG_WEEK_NUMBERING, with values like ISO8601 (default), american (?) etc.

[jotaen]

Transferring this to discussions. I’m not sure this is ever needed in practice, actually.