The "file" argument must be of type string. Received type object

Hi,

When I run `markdown-pdf README.md` I get the following error:

```
throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
^

TypeError [ERR_INVALID_ARG_TYPE]: The "file" argument must be of type string. Received type object
at validateString (internal/validators.js:125:11)
at normalizeSpawnArguments (child_process.js:414:3)
at spawn (child_process.js:553:16)
at Object.execFile (child_process.js:224:15)
at WriteStream. (/usr/local/lib/node_modules/markdown-pdf/index.js:117:22)
at WriteStream.emit (events.js:202:15)
at finishMaybe (_stream_writable.js:646:14)
at stream._final (_stream_writable.js:624:5)
at WriteStream._final (internal/fs/streams.js:270:3)
at callFinal (_stream_writable.js:617:10)
```

Can anyone suggest any solution?