function fs-svg should accept a function as an argument

[matbrady]

Would be convienent if fs-svg accepted a function for the $color arguement.

Use Case:

.example {
  background-image: url( fs-svg($name: 'chevron', $color: fs-color('blue') ) );
}

Pseudo:

@function fs-svg($name, $color) {
  // . . .
  // Fill
  $fill: '';
  @if ($color) {
    @if (type-of($color) == 'function') {
      $fill: call($color);
    } @else {
      $fill: ' fill="#{$color}"';
    }
  }
  // . . . 
}