Add a default border radius value to all border radius mixins

Created by: jeremyvii

It would be convenient if the $border-radius variable was the default value for all border radius mixins, like it is for border-radius(). That way you could include mixins, such as border-top-radius(), without providing a parameter.

I believe this would be an easy change and would be happy to open a PR. However, I wanted to make sure there wasn't a reason why there isn't already a default parameter for these mixins.

Before proposal

.item {
  @include border-top-radius($border-radius);
}

After proposal

.item {
  @include border-top-radius();
}