Please make the HighlightQuery class public

We're using the highlighter directly in our code, and the introduction of the package-scoped HighlightQuery in 4aa59aff0058b05e822592431eda4a469a6b9eef has made this impossible without building our own little fork.
Got it...sounds weird though that you use your own `FetchSubPhase` which depends on a custom `HighlighterContext`, as the context is pretty much bound to our `HighlightPhase`. I wonder now if its constructor should have been package private from the beginning too :) 

Anyways, I think a `protected` constructor would do the trick for you instead of `public`, I'd prefer that so it's clear that people are not supposed to instantiate a `HighlightQuery` in the ordinary usecase.