Adds convenience method to query if a particular PID is supported

[jimwhitelaw]

This is a convenience method that selects the correct supportedPIDS_xx_xx() query and parses the bit-encoded result, returning a simple Boolean value indicating PID support from the ECU.

This change includes an example program showing how to use the new method. Just call the new method with the PID you are interested in:

    uint8_t pid = ENGINE_RPM;
    bool pidOK = myELM327.isPidSupported(pid);