append subcommand does not work with zbarimg

The `pass otp append` command does not work with `zbar` i.e. following the instructions below from the README the password-store file is overwritten rather than appended to.

```
$ zbarimg -q --raw google-qrcode.png | pass otp append google/example@gmail.com
```

My qrcode file was a svg rather than a png if that matters.