Make it possible to specify a custom format for reporting broken links

[gromakovsky]

Clarification and motivation

xrefcheck is supposed to be used mainly on CI, e. g. GitHub Actions.
GitHub Actions has a special syntax for emitting warnings. AFAIU, it may look like this (image from the internet):

I think it would be neat to report broken links in such a way. And probably the best approach is to make xrefcheck accept some specification of a format in which it will report broken links.

Acceptance criteria

The user can run xrefcheck in such a way that it will report broken links in a user-defined format.
This format should be a function that takes:

• file name
• position in the file
• a message like Resource unavailable (404 Not Found)
• a link being checked
• maybe something else

and returns formatted text to be printed instead of the default message.