Handle unitless height and width from SVG

My workflow runs the SVGs through [svgo](https://github.com/svg/svgo/) via [svgo-grunt](https://github.com/svg/svgo-grunt) before grunticon, which cleans up and optimizes SVGs. One of its optimizations is to remove the default 'px' units, so this commit adds them back in to the CSS declarations if they're missing.