Add option tmpPath

Adding option for `tmpPath` so you can set an absolute root path for your `tmp` output.

To add some background, we had an issue on Windows where phantomjs was hanging when using the `tmp` folder set in the environment variable. It's really hard to reproduce, i would've shared an SVG file, but it's more random than that. It only happens on some machines and only if we have exactly 42 icons in the folder. If we add one or remove any of the others it does not hang. Then we found that it worked when we used a different `tmp` folder that's not under `%appdata%` so i added this option.