Feature request: add support for CUSTOM_ID

[marczz]

When we comment with org-remark an org mode buffer, if we have used an ID in the section or a parent, if we use org-remark-use-org-id it is linked from the annotation. This is quite handy when we are working on the source buffer, when the subsection where the annotation resides move because line where added before it, or because when refactoring the source we move the subtree, all the marks of the region and the line is lost, but thanks to the org id; we can find where the annotation is coming from, and restore it.

But using systematic org id on my sections, is not in my habits, I use on nearly each section a CUSTOM_ID and use it as reference for my internal links. I find the source more readable, with a meaningful name. Only when I truly need an unique ID among all my org files, I use an ID, as example org-transclusion need it.

So my feature request would be to also add an option to link to the closer CUSTOM_ID, working like the org-id feature.