Be able to test dynamic placeholders

[elicwhite]

It would be nice to have a way to test placeholders.

When generating placeholders in a loop like

$placeholder-name: 'foo';

@for $i from 1 through 4 {
  %#{$placeholder-name}-#{$i} {
    content: $i;
  }
}

it would be valuable to verify that it created them properly.