date: Rework argument processing

[RenjiSann]

This pull request addresses several points on the argument processing of date:

• Enfore the mutual exclusivity of --date, --file, --reference, and --resolution (not yet implemented).
• Successive --date, --file and --reference arguments override their previous introduction.

This is still work in progress, as I also would like to fix the distinction between FORMAT positional arguments (starting with +) and date positional arguments.