Add support for gruvbox_sign_column about gruvbox.nvim

[ellisonleao]

In the original gruvbox you can set based on relative colors (such as bg0) as well.

yep, that was the issue, I will add the original behaviour to this one just to keep consistency then.

[ellisonleao]

hey @timbedard, g:gruvbox_sign_column should be already supported as you can see it here . Do you have latest code over there?

[timbedard]

Pffft. Must have mis-grepped. I'll move the option to the setup function in my packer config and see if that does it.

[timbedard]

Ah. I see the problem. The way that get_color_from_var() works right now, you can only set colors based on the base palette. In the original gruvbox you can set based on relative colors (such as bg0) as well.