Various tweaks

[symphorien]

Hello, I want to package this plugin for nixpkgs, and since this required some patching and I had to make a PR to send the patches upstream, I figured I would add some more "cosmetic" changes. This results in this somewhat heterogeneous PR, I'll split it in several ones or drop commits if you want.

• enable logging without modifying the script by setting the SVED_DEBUG environment variable
• add a vim documentation file, heavily inspired by the existing README. I am open to rephrasing.
• don't print a message when quitting neovim if the exit code is 0. This is neovim specific because I use v:exiting.
• Don't call evinceSync as python3 evinceSync.py but /absolute/path/to/evinceSync.py and trust the shebang. This one is required by the specificities of nixpkgs, which relies one weird shebangs, but it should not harm other distros, so I sent it upstream anyway. Feel free to tell me if you would rather keep this patch downstream.

[peterbjorgensen]

Thanks a lot for the pull request. Your changes make sense.