Add https://github.com/aklt/plantuml-syntax to vim-plugins

Signed-off-by: Sirio Balmelli sirio@b-ad.ch

Motivation for this change

Wanted plantuml syntax highlighting in Vim

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS

  - system: `"x86_64-darwin"`
  - host os: `Darwin 16.7.0, macOS 10.12.6`
  - multi-user?: `no`
  - sandbox: `no`
  - version: `nix-env (Nix) 2.3`
  - channels(siriobalmelli): `"nixpkgs"`
  - nixpkgs: `/Users/siriobalmelli/.nix-defexpr/channels/nixpkgs`

  • other Linux distributions

   - system: `"x86_64-linux"`
   - host os: `Linux 4.15.0-45-generic, Ubuntu, 18.04.3 LTS (Bionic Beaver)`
   - multi-user?: `no`
   - sandbox: `yes`
   - version: `nix-env (Nix) 2.3`
   - channels(admlocal): `"nixpkgs, sirio.tar.gz"`
   - nixpkgs: `/home/admlocal/.nix-defexpr/channels/nixpkgs`

• [ n/a ] Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• [ n/a ] Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• [ n/a ] Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @