Add debug hook to trace feeds

[brentru]

Add virtual method to trace feeds

It would be nice if there was a virtual method that I could override by creating a subclass, where I could print out the name of the feed I had received, and the AdafruitIO_Data object I could display. It would be a convenient debugging hook. The default method would just have an empty body; I could subclass AdafruitIO_Group and override it. - https://forums.adafruit.com/viewtopic.php?f=56&t=139210

Proposed Method:

public:
    virtual void traceFeed(AdafruitIO_Data * d) {}

Implementation

class MyAdafruitIO_Group : public AdafruitIO_Group {
public:
    virtual void traceFeed(AdafruitIO_data * d) {
    Serial.println(d->feedName());
    ...maybe other stuff
};