adding disableStart option for Picker.Date.Range

The idea behind the option I am proposing, and which I have implemented through my fork, is to prevent users from inadvertently changing the start date field value. Without this option, in cases where the Picker.Date.Range object has been initialized to start at a certain date is likely to change if the user clicks more than once over the Picker.Date.Range control. With the implementation of the disableStart option, and when this option is set to true, the start date input from the Picker.Date.Range control is attributed readonly, i.e. disables the user from selecting this input field, and despite the number of clicks over the Picker.Date.Range control, the start date field's value will remain the initialized value.