feat: support custom name parser

[eatgrass]

since this forked repository is used as part of the NvChad, so I think it would be great to be able to highlight the NvChad Base46 theme color by name, by adding the theme's color table to the custom option, for example

local reload_colorizer = function()
  local base_30 = require("base46").get_theme_tb "base_30"
  local base_16 = require("base46").get_theme_tb "base_16"
  require("colorizer").setup {
    user_default_options = {
      custom = vim.tbl_extend("keep", base_30, base_16),
    },
  }
end
...
-- Lazy 
{
  "NvChad/nvim-colorizer.lua",
  dependencies = "NvChad/base46",
  config = reload_colorizer,
},

so that we can highlight those names accordingly, it's especially useful when editing the highlight groups