Refactoring of query-functions to support custom functions

[runarmyklebust]

I think it will be possible without too much work, to refactor the FunctionExpressionBuilder to use a QueryFunctionProvider-interface with the methods

String name()
org.elasticsearch.index.query.QueryBuilder build (interface functionArguments)

This will enable us and other users to implement their own query-functions by implementing this interface, thus make it really easy to add new functions in separate libs etc.

[sigdestad]

Sounds like a smart plan.