Add extended metadata support for DayOne Classic

[minchinweb]

Further to previous Pull Requests #350, #397, #739.

Closes #159. Closes #351. Closes #358.

Maintains extended metadata in DayOne entries when edited by jrnl. Export support for these have already been added.

This code has been used locally for probably 5 years now.

Checklist

• The code change is tested and works locally.
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• There is no commented out code in this PR.
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• Have you written new tests for your core changes, as applicable?

[micahellison]

Could you please add some tests for this functionality?

[minchinweb]

I added a basic test to make sure that the extended metadata is being added to new entries.

To do this, I added support for the extended metadata to the JSON exporter.

To completion's sake, extended metadata should also be added to the XML exporter, but I'm less sure on how to add subkeys, as would be needed.

Also, I discovered that we have no tests that cover editing an entry in the entire test suite. This is probably in part due to complexity (it would involve mocking opening an editor, changing the displayed file, and then closing the editor). One of the drivers of these changes was to make sure that extended metadata is maintained across edits, so at some point that would be good to add.

If you're satisfied with the additional test, it should be good to merge.

[micahellison]

This looks good. Thanks for adding tests! Could you resolve the merge issues?

[minchinweb]

@micahellison done!

[micahellison]

Thanks!