fix(#226): allow override hlgroups when background changes

[Bekaboo]

Fix #226.

Now the icon color variant will change correctly according to current background when user sets the background inside neovim process.

• Before this commit:

  1. On entering nvim: (correct)
  2. After :set bg=light light icon for light background :(
  3. After :set bg=dark dark icon for dark background :(

• After this commit:

  1. On entering nvim
  2. After :set bg=light :)
  3. After: :set bg=dark :)

[Bekaboo]

Thanks, please modify this pull request as needed, and hopefully it will be merge soon.

[alex-courtis]

Please address the linting failure: https://github.com/nvim-tree/nvim-web-devicons/blob/master/CONTRIBUTING.md

[Bekaboo]

Formatted with stylua.