Allow expression when setting a new dates for due and threshold ones

• Breaking changes:
  • deprecated function done (since 7.2.0) is removed. The new function done_with_config is renamed as done
  • Conf struct reorganization:
    • previously separated fields <name> and <name>_action are grouped into sub-structs in a form <name>: { <action>, <value> }
    • added field soon_days for calculating dates like soon+1d
    • all date fields (due and threshold) got extra feature: you can define a new value not only by setting an exact date, but by setting an expression that is calculated for each task. It makes easier to, e.g, push all due dates by a week by passing expression due+1w
• Added a lot of functions to deal with human-readable date expressions like due+1w or today+1m