Dynamic root dir #5

[AntoineGS]

Ability to change the root directory dynamically through a function (it can also be a string though that's less useful), with the following default:

  root_dir = function()
    return vim.fs.dirname(vim.fs.find(".git", { upward = true })[1])
  end,

Fixes #371
From the Readme:

root_dir

This allows changing the function that gets the root folder, the default looks for a parent folder that contains the folder .git.