Change to NTriples as a default for exporting RDF data

[CaptSolo]

Suggestion: change default RDF export format from RDF/XML to NTriples (because NTriples support streaming export while RDF/XML does not).

Motivation: in the SWIB15 Catmandu workshop there were a number of occasions when people (including presenters) forgot to specify "--type Ntriples" parameter resulting in long wait times (for full RDF model to be built in memory) followed by interrupting the process and repeating the export process with the right parameter.

It seems that RDF/XML is not a good default choice since novice users may not realize that the default option leads to unnecessary wait times. Workshop presenters also emphasized that NTriples is the best format to use when working with RDF data in Catmandu.

[nichtich]

Thanks for giving a good reason to implement this modification. Done in release 0.28 just pushed to CPAN.

[CaptSolo]

Thanks. That was fast! :)