feat: allow overrides to be function(colors)

This patch checks for table and function in `config.overrides` and
extracts overriden groups appropriate way. Also README.md is updated
to reflect this change.

README.md

@@ -128,6 +128,27 @@ require("gruvbox").setup({
 vim.cmd("colorscheme gruvbox")
 ```
 
+`overrides` can also be a `function(colors)` which returns table of overriden groups
+
+```lua
+require("gruvbox").setup({
+ overrides = function(colors)
+ return {
+ Title = { link = "GruvboxYellowBold" },
+ ["@comment"] = { fg = colors.gray, italic = true, bold = false },
+ }
+ end
+})
+vim.cmd("colorscheme gruvbox")
+```
+
+`colors` has those fields:
+ * `bg0`, `bg1`, `bg2`, `bg3`, `bg4`,
+ * `fg0`, `fg1`, `fg2`, `fg3`, `fg4`,
+ * `red`, `green`, `yellow`, `blue`, `purple`, `aqua`, `orange`, `gray`,
+ * `neutral_red`, `neutral_green`, `neutral_yellow`, `neutral_blue`, `neutral_purple`, `neutral_aqua`,
+ * `dark_red`, `dark_green`, `dark_aqua`.
+
 Please note that the override values must follow the attributes from the highlight group map, such as:
 
 - **fg** - foreground color

lua/gruvbox.lua

@@ -41,7 +41,7 @@ local Gruvbox = {}
 ---@field invert_tabline boolean?
 ---@field invert_intend_guides boolean?
 ---@field inverse boolean?
----@field overrides table<string, HighlightDefinition>?
+---@field overrides [table<string, HighlightDefinition> | fun(colors: table<string, string>): table<string, HighlightDefinition>]
 ---@field palette_overrides table<string, string>?
 Gruvbox.config = {
  terminal_colors = true,
@@ -1205,7 +1205,16 @@ local function get_groups()
  ["@lsp.type.variable"] = { link = "@variable" },
  }
 
- for group, hl in pairs(config.overrides) do
+ local overrides = {}
+ if config.overrides ~= nil then
+ if type(config.overrides) == "table" then
+ overrides = config.overrides
+ elseif type(config.overrides) == "function" then
+ overrides = config.overrides(colors)
+ end
+ end
+
+ for group, hl in pairs(overrides) do
  if groups[group] then
  -- "link" should not mix with other configs (:h hi-link)
  groups[group].link = nil