Implemented TagFilterCommand class
Reimplemented the Tagfilter filtering
Added Tree class with appropriate filter methods
Fixup: Added missing `end` kw for #filter in TagFilterCommand class
Added filter command comment
Added constructor for tag command class
Pulled filter helper methods one level up
Pulled the #filter_tree meth one lvl up, implemented CategoryFilterCommand
Added limit superclass method for querying the command if it allows searching in a certain dir
Added LimitRangeCommand class implementation
If we have no month or day, we use the first one, to search the complete year/month
Parse the start date and end date as Date objects, too
Implemented #search_in? method for checking if searching in a certain sub dir
Outsourced parsing of start and end dates in own method
Implemented LimitInCommand, which should inherit from LimitRangeCommand
Implemented LimitYearCommand class
Implemented LimitMonthCommand
Implemented LimitDayCommand
Started adding AddCommand implementation
implemented touch method