Refactored date-based filters to their own module.
Added a new date-based filter to look for stamps N days/weeks/months/years *after* a given date, for example:
"2017-06-29+4d" == "four days after the 29th june 2017" "2017-06-29+1m" == "1 month after the 29th june 2017"
and so on.