Sorry about that, considerd too straight forward. Simply add it to public section, just like in es8266.h:
public:
bool dataAvailable();
size_t readBytes(char *buffer, size_t length);
size_t readBytesUntil(char terminator, char *buffer, size_t length);
void print(const char *data);
void print(const __FlashStringHelper *data);
int peek();
void setTimeout(int timeout);
void flush();
void stop();
};