feat(options): cwd as function with nil for abort

[gjeusel]

Hello back ! Following the issue, the solution I came with.
Allowing to pass a function as the cwd, and when the function returns a nil, it should not trigger the rg run.

It can be used as follow:

require("cmp").setup({
  sources = {
    {
      name = "rg",
      options = {
        cwd = find_root_dir,  -- use rg only if find git in parent dir
      },
    },
  }
}

[lukas-reineke]

Thank you,

I still prefer this to be in cmp directly, like discussed in #27, but this might be a good workaround for the time being. 🤔
Let me think about this a bit.