Wrote a lot of comments, FIXMEs and TODOs.
As urgently as we need a proper rdf API/rdflib facade, we need to
re-evaluate commands.handlers and commands.arguments.
The latter, because we don't provide a nice way to define how we want
argument values to be validated/autocompleted etc., and the former because
this leads to really ugly code when it comes to using actual argument values
within command handlers.
Tasks:
  * write API for command argument type registration (validation regex/func/autocomplete)
  * find out if we can provide a way for handlers to access the actual argument subjects, not only their raw strings values. (rdf graph instead of graphname, file object instead of filename and so on..)