Add a minimum and maximum post ID option

[naxoc]

This is so the processing can be narrowed down

This is very simple for now. There is no UI to set the options, so that would need to be done from CLI, filtering, or in a pinch – the wp-admin/options.php page.

If the values are set, it would look something like this in the UI:

How to test

First, just verify that there is nothing new in the UI on wp-admin/admin.php?page=newspack-content-converter. Now set a minimum: wp option update ncc_min_post_id_to_process xx and verify that the minimum is shown (and respected).

Do the same with a maximum: wp option update ncc_max_post_id_to_process xxx and verify that it is shown and respected.

[gamebits]

This is functionally similar to the feature requested in https://github.com/Automattic/newspack-content-converter/issues/114.