Allow a variable to be attached to a topic

[Rémi Bèges]

The goal is to simplify the process for the user to make some variables remotely writeable

#include <Telemetry.h>

Telemetry TM;

void setup() {
  // Init the writeable parameter
  int32_t someParam = 0;

  TM.begin(115200);
  TM.attach_i32("foo", &someParam); // someParam is now writeable by publishing int32_t on topic "foo" 
}

void loop() {
  TM.pub_i32("feedback",myParameters.someParam);
  TM.update();
  delay(10); 
}