Improve `border-radius` on `options` object

This lets the user define another `border-radius` value easier if they don't want to use `$brand-round` without having to target `:first-child` and `:last-child`. For example:

```scss
// Old method
.options > li > a:first-child {
  border-radius: 6px 0 0 6px;
}
.options > li > a:last-child {
  border-radius: 0 6px 6px 0;
}

// New method
.options > li > a {
  border-radius: 6px;
}
```