Create public function for getting vertical rhythm units

[jdsteinbach]

Similar to Susy's gutter() function, TypeTuner needs a public spacing() function: this would probably replace the scale-spacing() mixin for most normal use.

[jdsteinbach]

But the responsive part… A simple function wouldn't output MQs. Perhaps another alias for scale-spacing() called scale-position() to write TRBL values would be the best short-term solution?

[jdsteinbach]

@mixin scale-position( $property, $trbl ) { 
  // output CSS
}