exporters: consolidate logic for output files

[mpollmeier]

for all 'single-outputfile-exporters': if the given outputFile is a directory: export into a new file in that directory otherwise: use the given outputFile as is, and create all parent directories (if not there already)

also: make each exporter define it's default file extension ... so that we can use exporters generically from the outside, while using their specific correct file extensions for the resulting files

[mpollmeier]

cc @DavidBakerEffendi - thanks for suggesting to default to .json|.xml rather than .graphson|.graphml