Support metric Units

[cocker-cc]

Is your feature request related to a problem? Please describe. Living in a modern and developed World, I am always frustrated when I have to deal with non-metric Units.

Describe the solution you'd like I'd like to be able to choose to display metric Units:

• [x] label_weight: kg
• [ ] label_bmi: kg/m²
• [ ] label_bmr: kJ/d and W (choosable)
• [ ] label_tdee: kJ/d and W (choosable)
• [ ] label_calories: kJ

For label_calories please use the String Energy and not Calories: not to confuse physical Quantity and Unit of Measurement.