refactor: move auto setting org_adapt_indentation to Config:extend

See #627 (comment)

ftplugin/org.lua

@@ -11,7 +11,6 @@ config:setup_mappings('text_objects')
 config:setup_foldlevel()
 
 if config.org_startup_indented then
-  config.org_adapt_indentation = not config.org_indent_mode_turns_off_org_adapt_indentation
   vim.b.org_indent_mode = true
 end
 require("orgmode.org.indent").setup()

lua/orgmode/config/init.lua

@@ -40,6 +40,9 @@ function Config:extend(opts)
     opts.org_priority_default = self.opts.org_priority_default
   end
   self.opts = vim.tbl_deep_extend('force', self.opts, opts)
+  if self.org_startup_indented then
+    self.org_adapt_indentation = not self.org_indent_mode_turns_off_org_adapt_indentation
+  end
   return self
 end