highcal: Show numerical year + month in header
highcal: Improve readability by using more colors
gitodo: Support for keywords "today" and "week"
gitodo: Make list of items unique before printing it
Use ISO 8601 weeks to identify weeks

http://www.ruby-doc.org/core-2.1.0/Time.html#method-i-strftime says:

ISO 8601 week-based year and week number:
The first week of YYYY starts with a Monday and includes YYYY-01-04.
The days in the year before the first week are in the last week of
the previous year.
  %G - The week-based year
  %V - Week number of the week-based year (01..53)