feat: add custom_highlights option

[danfry1]

Summary

Adds a custom_highlights option to setup() so users can override or extend any highlight group.

Accepts either a table or a function:

require("lume").setup({
  -- As a function (recommended) — gives access to palette colors
  custom_highlights = function(colors, variant)
    return {
      Normal = { bg = "#1E1F2E" },
      MiniDiffSignAdd = { fg = colors.accents.sage },
    }
  end,
})

The variant parameter is "dark" for now, future-proofed for when a light theme is added.

Overrides are applied after all built-in groups, so they always win.

Thanks to @iAmKevinCruz for the suggestion!

Closes #3