First implementation for date detection in string_search

[ephase]

This is my very first working work for search improvement. In this version, you can specify a date range for limit search results.

Usage (for now) is <start> <end> <search_string>, you can't indicate end date without start exemple :

khal search 01.01.2020 Dentist

[geier]

I like this, but perhaps we should make this more explicit by giving start and end as an argument, e.g. search --start 2022-05-01 Denstist. Then one could also give start without end.

[geier]

Please re-open if there is any interest in working on this.