add configTime, getLocalTime and support for time in Print

example:
```cpp
//done once on WiFi init
configTime(-7200, 3600, "pool.ntp.org");
//get local time
struct tm timeinfo;
if(!getLocalTime(&timeinfo)){
    Serial.println("Failed to obtain time");
    return;
}
//print time
Serial.println(&timeinfo);
//print time with different format
Serial.println(&timeinfo, "%A, %B %d %Y %H:%M:%S");
```
fixes: https://github.com/espressif/arduino-esp32/issues/29

cores/esp32/Print.h

@@ -83,6 +83,7 @@ public:
     size_t print(unsigned long, int = DEC);
     size_t print(double, int = 2);
     size_t print(const Printable&);
+    size_t print(struct tm * timeinfo, const char * format = NULL);
 
     //size_t println(const __FlashStringHelper *);
     size_t println(const String &s);
@@ -95,6 +96,7 @@ public:
     size_t println(unsigned long, int = DEC);
     size_t println(double, int = 2);
     size_t println(const Printable&);
+    size_t println(struct tm * timeinfo, const char * format = NULL);
     size_t println(void);
 };