Allow to customise redirect_url in config

[kant01ne]

Need to allow to customise redirect_url in configs:

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :intercom, ENV['INTERCOM_KEY'], ENV['INTERCOM_SECRET'], "https://custom.url/auth/callback"
end

[matsimitsu]

+1, this would make local development a lot less cumbersome.