Add context_base::arg()

include/fmt/core.h

@@ -1017,7 +1017,8 @@ class context_base {
 
  public:
   basic_parse_context<char_type> &parse_context() { return parse_context_; }
-  basic_format_args<Context> args() const { return args_; }
+  basic_format_args<Context> args() const { return args_; } // DEPRECATED!
+  basic_format_arg<Context> arg(unsigned id) const { return args_.get(id); }
 
   internal::error_handler error_handler() {
     return parse_context_.error_handler();