New ignore argument & config option

[mwouts]

Closes #986 Closes #974

TODO

• [ ] Find a more efficient way to determine if nb_file matches a glob than listing all the files in the glob
• [ ] We want the method to work also when nb_file does not exists!
• [ ] Find out how to do so in the context of the contents manager (path separator is always /, and there might be no underlying file system)