Won't work - just errors.

nodejs 10.9.0

```
> npm install -g markdown-pdf
> markdown-pdf test.md
internal/validators.js:125
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:394:3)
at spawn (child_process.js:519:38)
at Object.execFile (child_process.js:216:15)
at WriteStream. (C:\Users\username\AppData\Roaming\npm\node_modules\markdown-pdf\index.js:117:22)
at WriteStream.emit (events.js:187:15)
at finishMaybe (_stream_writable.js:641:14)
at stream._final (_stream_writable.js:619:5)
at WriteStream._final (internal/fs/streams.js:264:3)
at callFinal (_stream_writable.js:612:10)
```