[Feature Request] Add merging options

[nomandera]

I am requesting native support to handle merging existing and future exports be added.

For existing exports two or more previous exports would be combined into one new set devoid of duplicates. This obviously will require some sort of "safe" duplicate handling beyond simple filenames.

For future exports, setting the destination folder to an existing previous export should add new data only. I do not know if emails being moved being folders is a practical case to support when merging or if these would simply become unhanded duplicates.