Added missing defaults for config.prefix and config.cssSuffix

If you use dr-svg-sprites without grunt, defaults for `config.prefix` and `config.cssSuffix` are missing.
This causes multiple `undefined` in the css file.

![error](https://f.cloud.github.com/assets/992674/2368641/45bb48f6-a7b5-11e3-9419-e878ddaaf683.png)