Add "ModifyDateValue" action

This adds a generic action to modify a Date parameter (either of type Date or an arbitrary date string with a configurable format).

Configuration parameters:

• Format (defaults to Ymd as returned by Date input parameters
• Date interval - this will be passed to \DateTime::modify(), so all kinds of absolute and relative formats are allowed, e.g. midnight + 18 years for calculating when someone turns 18 from their birth date
• Include time - whether to include the time part in the output date value

Input parameters:

• Date - the actual date to modify