Thanks for mentioning Secret Agent. I had looked for a plugin to do that some time ago and couldn't find one. Now I have!
https://www.dephormation.org.uk/?page=81
Just bear in mind the default installation can break some pages (Soundcloud) or worsen your user experience (many web sites, Google, Wikipedia or Ars Technica among others, redirect you to the mobile version if you have a certain user agent).