replace: Completely merge given priority / date with existing

[inkarkat]

So that any combination of priority / date entered in the replacement will replace the corresponding original ones, but if they are left out, the original ones will be kept. In essence, omitted stuff will be kept, added stuff will override, only deletion of existing stuff is not possible (but this is replace, after all).

Fixes #386

Before submitting a pull request, please make sure the following is done:

• [X] Fork the repository and create your branch from master.
• [X] If you've added code that should be tested, add tests!
• [X] Ensure the test suite passes.
• [X] Format your code with ShellCheck.
• [X] Include a human-readable description of what the pull request is trying to accomplish.
• [ ] Steps for the reviewer(s) on how they can manually QA the changes.
• [X] Have a fixes #XX reference to the issue that this pull request fixes.