Add JSON tags to configuration struct

[kilianp07]

Hi !
It would be nice to add JSON tags to the conf structure to automate controller configuration. In my case this required me to create a structure with tags and cast it into your, I think this could be simplified by adding it to yours.

Here's my suggestion:

// ControllerConfig contains configurable parameters for a Controller.
type ControllerConfig struct {
	// ProportionalGain determines ratio of output response to error signal.
	ProportionalGain float64 `json:"kp"`
	// IntegralGain determines previous error's affect on output.
	IntegralGain float64 `json:"ki"`
	// DerivativeGain decreases the sensitivity to large reference changes.
	DerivativeGain float64 `json:"kd"`
}

Best regards.