Support the Turtle format as used by Wikidata

[niklas88]

Currently we need to convert Wikidata dumps to NTriple format before we can work with them which makes this unnecessarily complicated and slow. Instead we should directly support this common standard format directly. Parsing this is also a good way to gather some more parsing experience before rewriting the SPARQL parser.

Simplifying the Wikidata ⇒ QLever pipeline will also make it trivial to setup automatic weekly Wikidata updates.

[niklas88]

Fixed with merge of #140