enable users to control the behavior of the conversion tools

[hartig]

It should be possible for users to specify whether the RDF*-to-RDF converter adds the reified statements (in addition to the corresponding reification statements). The default behavior should be to add them.

Similarly, it should be possible for users to specify whether the RDF-to-RDF* converter expects the reified statements (and, thus, fails if any of them is missing).

[hartig]

In the context of addressing issue #18 I have added a boolean flag to RDFStar2RDF. The idea of this issue here now is to expose this flag as an argument of the command line tool, and to do something similar for the RDF-to-RDF* converter.