Add lunr options search customization

[meghan-ec]

Adds the search customization options b and k1 to the lunr per these docs https://lunrjs.com/guides/customising.html

[angeloashmore]

Thanks @meghan-ec, I'll review this as soon as I have a free moment. 👍

[jooola]

@angeloashmore @meghan-ec I feel this way of passing props downstream will limit too much the ability to extend the underlying engine configuration.

What about using hooks to configure the engine, pre/post index creation ?

Maybe this gives too much freedom to mess the plugin around, but this would allow user to pass plugins or configure languages.