Incremental flag in config

I run backstop with something like that:
```
const backstop = require('backstopjs');

backstop('reference', {config:'custom/backstop/config.json'});
```
I don't want to remove all reference images. There is a incremental param flag to do that `backstop reference --i` but I don't know how to do this with config.json file. The param `--filter` is similar.

Thanks for your help