use config file

[VladimirAlexiev]

@jazzyray:
#3 adds the use of a config file plantuml.cfg. I see the following issues:

1. It should check for the presence of such file, or should have an option. Should not die if a config file is not present.
2. hide empty methods etc are necessary for the diagram to look ok. So it's better to leave these in: they could be specified in the config file, but what's the benefit given that they must always be present?
3. Rather than concating the config file, it should use the -config option as per https://forum.plantuml.net/4266/configuration-file-specification (another option is to use #include inside the file, but I think it's slightly less convenient)
4. It should document the feature

[jazzyray]

The config file is in the repository. It will be cloned. I see no reason to check for existence. If it has been deleted something has gone wrong, why not die?

Do you want me to delete the config file that is in master, that is not used?
https://github.com/VladimirAlexiev/rdf2rml/blob/master/bin/plantuml.cfg

Will look at points 3 and 4.

[VladimirAlexiev]

Don't we want a config file per folder or per source file? In one case you may want eg rankdir=LR, in other cases you may want the default TB layout