Supporting type `self` or something that can represents the current instance.

[vilicvane]

When I am writing a declaration file, I found it really frustrating repeating the inherited method that returns the instance itself for a correct return type.

E.g. for animation library GSAP, many classes are inherited from class Animation, I have to repeat almost every method to update return type Animation to the current class.

class Animation {
    chain(): Animation;
}

class TweenLite extends Animation {
    ownProperty: any;
    chain(): TweenLite;
}

It would be great if we may use some type like self to make this easier, like this:

class Animation {
    chain(): self;
}

class TweenLite extends Animation {
    ownProperty: any;
}

[DanielRosenwasser]

Seems like a duplicate of #285.

[danquirk]

Also see #513