Added Power/Watt usage

[ZuluPro]

Added POWER usage from nvidia-smi.
I reproduced the same behavior than temperature pooling.

[tsengjun]

Hello @ZuluPro
How about getting temperature and power by a single command:
From
execlp("nvidia-smi", "nvidia-smi", "-l", "5", "-q", "-d", "TEMPERATURE", NULL);
To
execlp("nvidia-smi", "nvidia-smi", "-l", "5", "-q", "-d", "TEMPERATURE,POWER", NULL);
should get the same result.

Thanks!