Index in current working directory does not work on windows

* Hashdex version: 0.6.0
* Python version: 3.8.3
* Operating System: windows

### Description
```powershell
hasdex add --index photo-index.db \path
```
throws an exception about missing directory on indexer.py line 18

```powershell
hasdex add --index .\photo-index.db \path
```

Does work.
