Per mapping/object options

Features like https://github.com/nvim-treesitter/nvim-treesitter-textobjects/pull/150 and https://github.com/nvim-treesitter/nvim-treesitter-textobjects/issues/125 would benefit from being per-mapping/objects instead of global options.

Our current settings look like this

      keymaps = {
        ["af"] = "@function.outer",
        ["if"] = "@function.inner",
        ["ac"] = "@class.outer",
        ["ic"] = "@class.inner",
      }

We could extend them to something like

-- Global options are defined here
-- lookahead = true,
keymaps = {
  ["af"] = {
    textobject = "@function.outer",
    -- If these options aren't given the global/default ones
    -- are used.
    lookahead = true,
    auto_expand = true,
  },
  -- This form is valid, and it uses the global options.
  ["if"] = "@function.inner"
}

Start by locating the existing keymap and global-option handling in the plugin, then compare it with the linked textobjects pull request and issue. Define how per-mapping values override defaults while preserving the string form, and verify both forms and fallback behavior with the relevant tests.