Add Semantic Search support

[nhorton]

Per your ideas list, Semantic Search support would be really helpful.

We have rolled our own version at Unsupervisedcom inside our source, and we spend a lot of time tweaking it right now. The biggest issue is that most of the higher-level helpers, particularly ones like the Pagy extensions, don't play nicely with our custom efforts to do this, and it is problematic. Getting a base implementation in Searchkick that other things like Pagy could then start integrating over time would be very helpful.

I would be glad to share some of the code we have for this right now, or if you (@ankane ) have an architecture you have in mind for how to integrate it in that you want to share, we could even try to flesh it out using some of our code.