Output directory isn't created

If the output directory `bundles/` doesn't exist, I get the error

```
CRITICAL:root:Could not write file [workingdir]/bundles/styles.css
```

But If I `touch ./bundles/styles.css` before running the Squeezeit command, everything works. 

(I don't mind doing this workaround, but I thought this deserved submitting an issue.)
