support for ignoring/considering some paths

[yarikoptic]

I am not certain yet it should be just a list of some regexes or relative paths. Most likely it would need to be both. E.g. considering git repositories (or DataLad datasets): we can immediately ignore .git/objects and probably .git/logs and some others if we are only interested in status. But also we might like to ignore anything git ignores (which could come from .gitignore or some other additional ways to specify those ignores)! So we might want to consult git on that somehow to ideally discover directories to skip etc.