create_matches function takes 1.4 s on large nix files

[create_matches](https://github.com/ap/vim-css-color/blob/master/autoload/css_color.vim#L226-L244) takes 1.4s according to profiling when opening large nix files like [all-packages.nix](https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix) and slows down the overall editor performance for inserts, moves, deletes, etc to seconds of input delay.