[Feature Request] Calculate and show power draw

[tanius]

Currently, AccA will show both the instantaneous voltage and current. However, for an end user, the instantaneous power draw in Watts or milliwatts would be more intuitive and useful. I propose to include this in the program output alongside voltage and current. It's easily calculates as "voltage × current = power". For example, 4 V and 0.5 A means 2 W power draw.

The reason this is more intuitive and useful for end users is that power values are easily comparable, while current values are only comparable if the voltage stays constant. But voltage drops during a discharge. So at the beginning of a discharge, 500 mA × 4.2 V = 2.1 W while towards the end 500 mA × 3.0 V = 1.5 W. When only looking at the current draw, the user would assume the device is consuming the same in both cases, while actually it consumed more in the first case and less in the second.

[VR-25]

@MatteCarra, acca --info also outputs power_now (in Watts). It's just a matter of parsing that line - no need to calculate anything.