let the user give an 'ignored_files' option for the checkers

Hi project maintainers,

for an application I have, I really would like to discriminate files per rule: for <%= reasons %>, I have migrations files with very long lines, and this is polluting the output, but I still want to run the other rules on them.

That is the reason I started this branch and I would like your opinion on the idea as well as the implementation before going further because I wasn't totally sure of myself.

I would love some feedback <3

Best Regards,
Lucas