Multifield highlighting hacked into FVH

Proof of concept for #3750
Multifield highlighting hacked into Simple

Doesn't work for requireFieldMatch = true at this point.

Proof of concept for #3750
More multi-field highlighter work.

1.  Back out the plain highlighter hack.  Too nasty.
2.  Plumb a field to turn it on.
3.  Decent start on test scenarios.
4.  More work on FVH doing the right thing.  Big thing here is that boosts
on child terms affect the child terms.