Specific rule to check private member variable

[keineahnung2345]

We follow this C++ coding style rule, and its 11th rule says that private member variable name should be like length_. But currently all varaibles are in the same group, so I can't make only "private member variable" be checked by its specific rule.

Also, can you please add a rule of "lower_snake_"(lower_snake + underscore), thanks!