Use generic color names from palette in overrides and other plugins setup code

[darkangel-ua]

I did couple of tries and came with this design. Now we can use

    local colors = require('gruvbox').palette
    require("gruvbox").setup({
        contrast = "hard",
        overrides = {
            SignColumn = { bg = colors.bg0 },
            GruvboxRedSign = { bg = colors.bg0 },

as well as

    local colors = require('gruvbox').palette
    require("bufferline").setup({
        highlights = {
            fill = {
                bg = colors.bg0,
            },

and bg0 will be correct in both cases and for any background/contrast combinations we use.
#306

[ambroisie]

I'm quite interested in making use of this, or any solution which exposes the plug-ins palette rather than requiring hard-coding of colors in setup.