Add option for max files size for autoFiles

This is my attempt to add the ability to specify a maximum size when using the `autoFiles` option. This works like the `maxFieldsSize` option in that the `maxFilesSize` option counts the combined bytes of all the files.

Note that the file cleanup was moved to a `destroyFile` function, which will correctly remove the file if the `fd` has not closed by that time and keeps it from `ENOENT` errors when the `fd` had never opened before the `destroy` call.

closes #26