Add extension to `Known Custom Cops`
Add extension to `Known Custom Cops`

As discussed here: https://github.com/bbatsov/rubocop/pull/5411#issuecomment-356868398
scottmatthewman/rubocop,haziqhafizuddin/rubocop,petehamilton/rubocop,deivid-rodriguez/rubocop,pocke/rubocop,akihiro17/rubocop,bbatsov/rubocop,rrosenblum/rubocop,sue445/rubocop,meganemura/rubocop,bquorning/rubocop,vergenzt/rubocop,haziqhafizuddin/rubocop,vergenzt/rubocop,sue445/rubocop,maxjacobson/rubocop,panthomakos/rubocop,jfelchner/rubocop,sue445/rubocop,deivid-rodriguez/rubocop,tdeo/rubocop,smakagon/rubocop,tdeo/rubocop,bquorning/rubocop,bquorning/rubocop,palkan/rubocop,urbanautomaton/rubocop,maxjacobson/rubocop,jmks/rubocop,akihiro17/rubocop,bbatsov/rubocop,jmks/rubocop,deivid-rodriguez/rubocop,tejasbubane/rubocop,petehamilton/rubocop,panthomakos/rubocop,meganemura/rubocop,urbanautomaton/rubocop,smakagon/rubocop,jfelchner/rubocop,akihiro17/rubocop,palkan/rubocop,petehamilton/rubocop,scottmatthewman/rubocop,mikegee/rubocop,jfelchner/rubocop,jmks/rubocop,palkan/rubocop,panthomakos/rubocop,tejasbubane/rubocop,tejasbubane/rubocop,rrosenblum/rubocop,pocke/rubocop,mikegee/rubocop,rrosenblum/rubocop,mikegee/rubocop,vergenzt/rubocop,tdeo/rubocop,maxjacobson/rubocop,meganemura/rubocop