fixed: When todo-comments.nvim as optional and report an error

[yibuma]

When todo-comments.nvim is optional, the first time it is used vim.defer_fn does not block and returns immediately, causing later code to read config.options as nil.

[yibuma]

How to reproduce.
Using packer.nvim configuration

-- Lua
use {
  "folke/todo-comments.nvim",
  requires = "nvim-lua/plenary.nvim",
  config = function() require("todo-comments").setup {} end,
  cmd = { 'TodoTelescope' },
}

The first time you execute :TodoTelescope you get an error.

[Weissle]

This commit may fix this issue. #138