Add `Doc:get_indent_info`

It returns the indentation type, size and confirmation status, used by 
the `Doc`.

data/core/doc/init.lua

@@ -114,6 +114,14 @@ function Doc:clean()
 end
 
 
+function Doc:get_indent_info()
+  if not self.indent_info then return config.tab_type, config.indent_size, false end
+  return self.indent_info.type or config.tab_type,
+         self.indent_info.size or config.indent_size,
+         self.indent_info.confirmed
+end
+
+
 function Doc:get_change_id()
   return self.undo_stack.idx
 end