Add a confirmation to capistrano deploys

deploy.rb

set :confirm_keys, lambda {
  puts "This is the config file that will be uploaded and used in production:"
  puts IO.read('config/deploy/production-secrets.json')
  ask :answer, "Continue deploying with these config settings? [Y/N]"
  fetch(:answer).downcase.strip == 'y'
}

namespace :deploy do
  desc "Confirms if the config file should be uploaded"
  task :prompt do
    unless fetch(:confirm_keys)
      puts "Deploy cancelled\n"
      exit
    end
  end
end

before :started, "deploy:prompt"