Add .rubocop_todo.yml

[dentarg]

@olleolleolle expressed it so well in sinatra/sinatra#1537 (comment) so I'm just going to leave his comment here:

RuboCop has a concept of a TODO file - a list of remaining issues to be skipped. Having such a file would allow us to keep enabled the rules we want to follow for new code, while at the same time allowing the old code not to trouble that effort.

If some of the rules which didn't pass right now are desirable to pass in the future, we could use rubocop --auto-gen-config to create and include a file .rubocop_todo.yml, oh, I blogged about this workflow.

Related to #134