Move transcode path to predefined temp directory

To me, it seems to make more sense to store temporary files such as transcodes within the system's defined temp directory, rather than one relative to the project root.

In the case of Windows, this would use a path like `C:\Users\Matt\AppData\Local\Temp`, and on UNIX, `/tmp/wavebox`.

EDIT: clarification, this will use the user's defined temp path first (as on Windows), but fall back to the system temp if there isn't one (as on UNIX).