Configurable `id` property on full-text search: Lunr2 integration

[Letrab]

As documented in https://github.com/itemsapi/itemsjs/blob/master/docs/lunr2-integration.md it is possible to use Lunr2 for full-text searching.

I want to try to use this, because I need nested/deep object search (which is possible via the extractor of a field https://lunrjs.com/docs/lunr.Builder.html#field )

But, my data set has no id property, but e.g. a unique property uuid. So it would be nice to make this property configurable in order to be more flexible on this.

[cigolpl]

It's done. More details in PR tests or integration examples in docs.