feat: add extra_text_objects option

[shebpamm]

Add a new optional config option extra_text_objects which is merged with default_text_objects.

The use case is to allow users to configure third-party text-object mappings they've declared somewhere else. For example I'm adding my own treesitter text-objects and the any quote (q) from targets.vim.

Tested with following config:

      require("leap-spooky").setup {
        extra_text_objects = {
          "iq", "aq",
          "iv", "av",
          "ik", "ak",
        },

[ggandor]

Thanks!