Make use of Jena's extension rsparql extension mechanism

[Aklakan]

Jena already provides SPARQL command line tooling with extension support. As sparql-integrate does a bit more than just registering extensions (e.g. it supports processing sequences of queries from multiple files), it does not seem possible to integrate completely into (r)sparql (i.e. having sparql-integrate as only a command line wrapper that invokes (r)sparql with the appropriate extensions).

Yet, there are 2 things we can do:

• Make it possible to use sparql-integrate extensions via jena; which means documenting the namespaces for the extensions and testing it with jena's native command line tooling
• Extend sparql-integrate to support the same extension points as (r)sparql