kingfisher_msgs: Add fields for user current monitoring, LED dimming

[mikepurvis]

Suggested:

# Instantaneous current drawn by each motor, in amps.
float32 current_left
float32 current_right
float32 current_user
float32 power_consumed  # watt-hours

These should go in for indigo; they support new features of the 2014 MCU.

Note that future firmware may also include capability to set running light brightness (eg, dimming them at night to save power, turning them off to avoid colregs violation). If a message must be modified to support that feature, it should be done at this time as well.