Update @mixin flex() to accomodate box-flex

So the previous pull request #11 was actually broken.

The output of `@mixin flex()` when given a list wasn't filling out all the properties correctly.

This PR should fix that, and ensure that all box-flex properties are filled out appropriately too!

Now both, for example, `@include flex(10, 5, 100px)` and `@include flex(10 5 100px)` work!