change param "std::string" to "const std::string&"

[wqvbjhc]

change param "std::string" to "const std::string&"

[zsummer]

Sorry for my late response.
Your push is useful and thanks very much.
But I have some problem: In the interface, I need "std::string" not "const std::string &" because sometimes I call this interface like "Config("config.cfg")" or I only use it with std::string type.
so . maybe you need adjust the problem and repush it, very thanks!
best regards.