Disallow units for zero values

[pure-js]

Please add if statement for background position value, and if value is zero write it without px.
At this moment generated code looks like this:

$icon_assistant_x = 0px;

But will be excellent, if it would look like this

$icon_assistant_x = 0;

[twolfson]

This is a duplicate of twolfson/grunt-spritesmith#156, twolfson/grunt-spritesmith#89, twolfson/spritesheet-templates#43, and twolfson/spritesheet-templates#11.

This request is entering CSS optimization territory and encourage developers to use another CSS optimization task after spritesmith to handle adjusting rules (e.g. 0px -> 0). Here are some tools to evaluate:

• https://www.npmjs.com/package/grunt-csso
  • Wrapper for http://bem.info/tools/csso/
• https://github.com/begriffs/css-ratiocinator