Make Thenable assignable to PromiseLike

[Strate]

Add overloads to Bluebird.Thenable.then method to correspond with ES6 PromiseLike interface.

See also #29 (comment)

[louy]

Any reason for not using extends PromiseLike<R>?

[Strate]

@louy PromiseLike.then returns PromiseLike, not Bluebird.Thenable

[Strate]

But, maybe we could just alias Thenable to PromiseLike.

[blakeembrey]

@louy You shouldn't use extends PromiseLike because this definition will be used in a non-ES6 environment.

[Strate]

@blakeembrey PromiseLike exists in lib.es5.d.ts

[blakeembrey]

@Strate The perhaps can it can be used then, I had never noticed that. I'm always a little wary, since they can change at any time. For instance, if the break that did occur happened, that would have meant it would break when updating TypeScript randomly. It's not a lot of fun to track TypeScript versions for testing.

[Strate]

@blakeembrey To avoid incinsistences between typescript's definition of PromiseLike, and bluebird's definition Thenable it is possible to alias Thenable to PromiseLike. But, Bluebird class should have overloading for each .then declaration to fix return type of that method.

BTW, I think this should be discussed separately.

[blakeembrey]

@Strate I agree. I don't see anything wrong with the current change 👍