Provide generic control service

[Aerijo]

Hey @izuzak, @allefeld and I have been drawing up a PDF viewer service intended to make it possible to use any viewer that implements the service.

We've agreed on a final draft, and were looking for input from another potential viewer that might implement the service. We've also reached out to atom-latex to see if they are interested in consuming the service, but I hope for it to enable other packages to take advantage of it too (even if it's just other LaTeX packages).

If you're interested, I or someone else will probably make a PR to implement the final spec here once it is finalised.

[izuzak]

Hey @Aerijo @allefeld 👋 Thanks for letting me know and for working on this in the first place :bow: And I'm sorry for taking this long to respond -- health issues and the current coronavirus situation have kept me away from personal projects 😢

I think the two of you and your packages are waaaay ahead of me and this package. So I'm 👍 on whatever you decide on and I'd be happy to merge PRs if you decide to open them.

Thanks again and take care! 🙏