Per mapping/object options

[stsewd]

Features like #150 and #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"
}

[ilan-schemoul]

I also think it is relevant for another use case : I added a custom @type to my custom files so I can select the type of a declaration for example in int i = 8 I can select int. Here I clearly prefer lookbehind than lookahead but do not want to modify global settings.
I therefore made a PR #588