Scripting: Add script engine for lucene expressions

Adds support for new "expression" script engine.

This is backed by lucene expressions.  These are javascript expressions, which can only access numeric fielddata, parameters, and _score.  They can only be used for searches (not document updates).

closes #6818
I agree with @rmuir. If we make it optional, we would have to do some magic to make the expression script engine load only if expressions are available (which I guess isn't that hard to do, but the other built in script engines don't assume they are optional).