Better Sass Media Queries

```
@media (min-width: $screen-sm-max: ($screen-md-min - 1) !default;
$screen-xs-max: ($screen-sm-min - 1) !default;)
```
would set up for something a little easier to read, such as when making a media query:
```
/* SM */
@media (min-width: $screen-sm-min) and ($screen-sm-max)
```

Same could end up going for CSS variables