[core] Refactoring IOFormats so there is one and only obvious way to send it.

This is the commit where I admit that having more than one input/output
format for readers and writers was complicating the code too much for a
very small gain, and that it would be easier to only have one way to do
it.

So such way is now:

- Returning (or yielding) a dict if you have key-value type collections.
- Returning (or yielding) a tuple if you have a list-type collection.
- Returning (or yielding) something else otherwise, which will continue
  to work like the old "arg0" format.

IOFORMAT options has been removed in favour of a RemovedOption, which
will complain if you're still trying to set it to anything else than the
one value allowed.

tests/test_basicusage.py

@@ -12,4 +12,5 @@ def test_run_graph_noop():
 
     with patch('bonobo._api._is_interactive_console', side_effect=lambda: False):
         result = bonobo.run(graph)
+
     assert isinstance(result, GraphExecutionContext)