Please support beatiful output in the PROV-N serializer

[olebole]

The PROV-N serializer is a nice tool for writing documentation and doctests (apart from #130) for provenance applications. However, it would be nice if it would support nicely formatted output by wrapping long lines, as it is done in the PROV-N Standard:

wasDerivedFrom(ex:d; e2, e1, a, g2, u1, 
               [prov:type='prov:Revision', 
                ex:comment="a righteous derivation"])

Currently, the output is just one line here, which is much harder readable, and not suitable to directly include in documentation:

wasDerivedFrom(ex:d; e2, e1, a, g2, u1, [prov:type='prov:Revision', ex:comment="a righteous derivation"])