RML to SPARQL should support adding prefixes to sparql queries

[Aklakan]

There are the following ways forhow this can be handled and appropriate configuration options should be provided:

• repeat prefixes on every query (gets ugly when many prefixes are involved)
• only add the prefixes to the beginning of a file containing sparql queries (our parser can handle this, but probably less portable)

For the CLI, in addition it may be worthwhile to add an option that outputs each query to a separate file.