Make annotation probability configurable via a parameter

A surface form is currently linked if its total score is > 0.5. This value should be configurable via a annotation threshold REST parameter (similar to support in the Lucene model). A lower value would show more annotations, a higher value would only show annotations the system is very confident about.

The relevant line in DBSpotter:

- https://github.com/dbpedia-spotlight/dbpedia-spotlight/blob/master/core/src/main/scala/org/dbpedia/spotlight/db/DBSpotter.scala#L114