Adds submitFullDate option

This adds a convenience option called 'submitFullDate' to the editableOptions object. I've had a pretty hard time adjusting for time zones when x-editable submits dates. This option makes sure to submit an ISO String version of the date when set to true by sending the x-editable library a params function. 

Not sure if you consider this a valuable or valid feature for what this binding does. This binding has been super helpful but one of my pet peeves is creating otherwise unnecessary observables to pass through to x-editable, and I thought this would be succint and convenient.