features(retnia): Add retnia calculations.

This PR adds calculations needed to write a mustache template that supports retnia. 

I came across the need for this in a recent project. I used a template like the one below, but since mustache templates don't support JS functions, I needed to do the calcs elsewhere. 

Example Template: 
```
{{#items}}
.retnia_sprite_{{name}} {
  background-image: url('{{{escaped_image}}}');
  background-position: {{retnia.half_offset_x}} {{retnia.half_offset_y}};
  width: {{retnia.half_width}};
  height: {{retnia.half_height}};
  background-size: {{retnia.half_total_width}} {{retnia.half_total_height}};
}
{{/items}}
```

@note It could be worth building this out a bit more to add robust retnia support to spritesmith. ie: adding documentation, example templates, etc.