How would you feel about applying the filter after reading from the `opts` hash? Something like this:

    @readline = ReadlineBlankFilter.new(opts.fetch(:readline, Readline))

In the integration specs we inject a `FakeReadline` instance into `CLI` (see this helper: https://github.com/thoughtbot/gitsh/blob/master/spec/support/gitsh_runner.rb#L43). To make sure those specs still exercise as much of the whole program as possible we should make sure they use the `ReadlineBlankFilter` somehow, and applying it to the injected `Readline` here avoids complicating the already slightly hairy integration spec setup even further.