Create output folders for spricon

Instead of making new promises above, could just wrap this in the mkdirp

```
mkdirp(cssPngOutput, function (error) {
    if (error) {
        console.error( "Error creating directories: " + cssPngOutput);
        console.error( "Error: " + error);
        return svgCssDefer.reject(error)
    }
    fs.writeFile(
        cssSvgOutput, '@if ($svg-support) {\n' + svgCss.join('\n\n') + '\n}\n',
        function () {
            svgCssDefer.resolve();
        }
    );
});
````