Adds option to include dimensions in generated CSS files

This is a redo of a [previous pull request](https://github.com/filamentgroup/grunticon/pull/32/files) by @davidlwarner, just on the more up-to-date code base.

This adds a new option called `cssdimensions`. It defaults to `false` which changes nothing about the current output. But when set to `true`, it will include `height` and `width` properties in the outputted CSS files and remove the inline style height and width properties in the generated `previewhtml`.

I took my best stab at this. Please let me know if I am missed adding code to any other files or if I added code to files unnecessarily.