[bz5403069] Support cookie getSub/setSub

[add0n]

The getSub/setSub methods are very useful to read and write hashed key/value pairs. Please support this so the interface is more similar to the YUI3 Cookie class.

[caridy]

mojito-cookie-addon for the client side is already using Y.Cookie for all these methods:

        [
            'exists',
            'get',
            'getSub',
            'getSubs',
            'remove',
            'removeSub',
            'set',
            'setSub',
            'setSubs'
        ]

We just need to get to feature parity for the server side implementation.