Feature: rule verification

[PaulSonOfLars]

It would be nice if the linter was able to check whether a rule is defined or not. This would likely have helped avoid #116, where rules seemed to be ignored because of trailing whitespace.

This should be used to check the rules specified in .add and .remove in the config, as well as protolint:disable directives.

[yoheimuta]

@PaulSonOfLars Thank you for making a nice feature issue! That sounds useful.