Omitted --output-path is not behaving in the way --help suggests (current working dir vs. input dir)

Tanks for this useful tool!

--help says:
```
-o, --output-path <FOLDER>          Directory to save output to. If not given, input file directory is used
```

But when omitting this flag, the current working directory is used instead of the input file directory. At least that's what happened on 0.2.3 that I've just installed from crates.io.