Add Promise.prototype.finally per tc39 0716

[clavecoder]

Since TC39 moved Promise.prototype.finally to Stage 2 in the 07 '16 meeting, add to core.js. Was requested previously as #70, by others.

[zloirock]

Makes sense. Feel free to add a PR.

[xjamundx]

Any movement here?

This one seems alright:
https://github.com/matthew-andrews/Promise.prototype.finally/blob/master/finally.js

[zloirock]

@xjamundx now I have some problems with time, it will be added when I'll be a little freer.

[clavecoder]

I'll integrate this weekend, promise!

[zloirock]

@clavecoder 👍

[xjamundx]

Thanks friends!

[Jessidhia]

I'd suggest basing it on the proposal's own polyfill at https://github.com/tc39/proposal-promise-finally/blob/master/polyfill.js. It takes one less microtask tick to settle and correctly supports @@species (though you may be able to use core-js's _species-constructor).

[clavecoder]

PR (#238) created, as promised. Tripped up a bit, perhaps, with the reserved word function name and, possibly, by the species of Promise. Preliminary reviews very welcome!

[zloirock]

ed59a9c, 86ed553, fd70e24, 0bc7a9e

[clavecoder]

Thanks for bringing it home!