Feature: Add support for Week ISO

[xavierkh]

Hi,

It would be interesting to also support the ISO WEEK numbering, already handled with %V flag in strftime

This would make issue titles consistent with business calendars

Done in periodictask 3.1.1

lib/scheduled_tasks_checker.rb:11:      subject.gsub!('**WEEK**', Time.now.strftime("%W"))
lib/scheduled_tasks_checker.rb:12:      subject.gsub!('**WEEKISO**', Time.now.strftime("%V"))
lib/scheduled_tasks_checker.rb:17:      description.gsub!('**WEEK**', Time.now.strftime("%W"))
lib/scheduled_tasks_checker.rb:18:      description.gsub!('**WEEKISO**', Time.now.strftime("%V"))