add sub-word matching in elasticsearch

[hpfast]

I want to implement search-as-you-type and/or partial matching for an application. The elasticsearch guide's description seemed a useful starting point to me, using edge-ngrams.

Would you be open to adding such a capability? Probably on a PiTs API (to be created, see #54), and I would like to try to implement this but would need some pointers where in the codebase to set up the ES filters (probably in core or schemas?). And any concerns/suggestions? Would we want to put it behind a flag?