Use mixin for border-radius

I would be better to start using mixins for options styling attributes such as `border-radius`, like so:

**variables.scss**
```
$enable-rounded: true; // or false
```

**Mixin**
```
@mixin border-radius($radius: $border-radius) {
  @if $enable-rounded {
    border-radius: $radius;
  }
}
```

Meaning you won't have unecessary CSS bloat like `border-radius: 0;` when it's not needed
