Enabling printf style output method with variadic arguments

[aentinger]

Usage example:

void setup() {
  Serial.begin(9600);
}

static int loop_cnt = 0;

void loop() {
  Serial.printf  ("[%d] ", millis()); 
  Serial.printfln("Loop Cnt %d", loop_cnt);
  loop_cnt++;
  delay(1000);  
}

Output:

[0] Loop Cnt 0
[999] Loop Cnt 1
[199] Loop Cnt 2
...

The function can't be named print because in this case its signature overlaps with size_t print(const char[]) (No variadic arguments is a legimitate use case).

[CLAassistant]

All committers have signed the CLA.

[matthijskooijman]

This seems like a duplicate of #28, which IMHO has a better implementation that has no fixed-size buffer. So I'm going to go ahead and close this PR, to keep things a bit more tidy (also since this feature is a bit controversial, as discussed in #32 and #28).