Expose a (JENA) SPARQL Extension

While thinking about certain data integration tasks, I had the idea to just do everything using (Jena) SPARQL extensions, such as done at [this project](https://github.com/AKSW/jena-sparql-api/tree/develop/jena-sparql-api-sparql-ext). Note, that Jena's extension system supports maven dependencies to register their own SPARQL extensions simply by including them - no further code necessary. This works by specifying a start-up class in the file `src/main/resources/META-INF/services/org.apache.jena.system.JenaSubsystemLifecycle`.

In principle, there could be a limes integration, where a pseudo SERVICE url is used to invoke limes.
The body of the LIMES service would contain the configuration, i.e. the two concepts which to interlink, the properties to base the metrics expression on, the metric expression, and the threshold.
In principle it could look something like:

```
SELECT ?x ?y {
SERVICE {
SERVICE { ?x a :Airport ; rdfs:label ?xl }
SERVICE { ?y a :Aerodrome ; rdfs:label ?yl }
FILTER(limes:trigrams(?xl, ?yl) > 0.9)
}
}
```

What do you think about this?