Added enable(), disable() and option (disabled = true/false)

Love your colorpicker.  Needed a little more on enable/disable. 

Added disable and enable functions as well as an option for enabling or disabling the control

option is "disabled: true/false" (default false).

spectrum('disable') to disable control
spectrum('enable') to enable it

Seems to work ;-)

 I hope this code is helpful to you.