Problem: Current implementation of the markdown scanner is hard
to extend, so we need to refactor it to add support for new annotations.

Solution: Refactor; improve handling annotations, remove IMSAll state
as it's not required, rename functions.

Problem: Current implementation of the markdown scanner is hard
to extend, so we need to refactor it to add support for new annotations.

Solution: Refactor; isolated processing annotations for different
types of nodes.

Problem: Current implementation of the markdown scanner is hard
to extend, so we need to refactor it to add support for new annotations.

Solution: Refactor; isolate processing annotations for different
types of nodes.

Problem: Currently xrefcheck is not able to detect possibly bad
copy-pastes, when some links are referring the same file, but
from the link name it seems that one of
that links should refer other file.

Solution: Implement check, add support for related annotations
for `.md` files, add corresponding settings to the config.

Remove extra parameters in md scanner

Problem: Currently xrefcheck is not able to detect possible bad
copy-pastes, when some links are referring the same file, but
from the link names it seems that one of
those links should refer other file.

Solution: Implement check, add corresponding settings to the config.

Problem: Currently xrefcheck is able to detect possibly bad
copy-pastes, but there is no way to disable those checks
locally for a file/paragraph/link.

Solution: Add support for related annotations for `.md` files.

Review: style, typos, naming

Review: formatting, partition

Review: fix config, README, CHANGES

Fix tests