Provide configuration file

As per #3 we would need to provide a list of warnings to be checked and values for threshold on command line. This will make for very long command line. We can just place a warnings-plugin.json or .yml (to keep some resemblance to gitlab-ci) and make it even hidden (.warnings-plugin), which can later be taken without location argument (look at current directory and recurs through childs in case something is there).