Whitelist for trivial accessors

[markijbema]

I think it would be helpful if one could define a whitelist for the trivial accessors cop. for instance the following methods typically aren't accessors: to_hash, to_s. I think we could add some of those default methods to the whitelist by default, but for instance a software package (in this case I'm trying rubocop on discourses oneboxes) might define similar methods (in this case to_html), so a whitelist would be really helpful.

This problem is partly addressed by the same name option, but I think with a whitelist you can keep rubocop stricter, while allowing these few methods.

[markijbema]

(looks like a simple enough fix, creating a pull request)