[26] Allow to exclude service users from commit checks

This is an implementation for https://github.com/sford/yet-another-commit-checker/issues/26.

We use this to allow Jenkins to commit without a proper JIRA e.g. for automatic code formatting done by Jenkins.
In my testing, this throws an NPE if the `excludeServiceUserCommits` setting doesn't exist. `getBoolean` supports default values pretty easily, so I will fix after merge.