Add Arrays::Flatten

> You're welcome @plapier
> https://groups.google.com/forum/#!searchin/sass-lang/compact/sass-lang/nF6Ux3aGJ1I/ommkuEB7nCMJ

```scss
flatten( (1, (2, (3, (4, 5)), 6), 7) ) => 1, 2, 3, 4, 5, 6, 7
```