Maybe it's better to make the mixing accept 2-element list for `$size`. It would allow us to keep the current syntax:
```scss
@include triangle(10px, black, down);
```
…more naturally but also to use it like this:
```scss
@include triangle(10px 20px, black, down);
```
Besides, it's a little more consitent with the way we pass arguments to other mixins ([size](https://github.com/thoughtbot/bourbon/blob/master/app/assets/stylesheets/addons/_size.scss) is a good example).

@P233 What do you think about it? :)