improve time categorization directives
Felipe Sateler writes at http://bugs.debian.org/751280:
A very useful function would be that arbtt could tell me what I did today, this week and this month. Unfortunately, this is not possible, since categorize.cfg does not know about the current date. Moreover, the TimeDiff type can only deal with spans of 99 hours long.
So, for full flexibility I think the following is necessary:
- Add a $now variable that represents the Date arbtt-stats is being run.
- Add a "week of month" function to Date.
- extend the TimeDiff spec to allow unlimited number of digits before the ':'.