[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_commands.py

@@ -30,9 +30,13 @@ def test_entrypoint():
     for command in pkg_resources.iter_entry_points('bonobo.commands'):
         commands[command.name] = command
 
-    assert 'init' in commands
-    assert 'run' in commands
-    assert 'version' in commands
+    assert not {
+        'convert',
+        'init',
+        'inspect',
+        'run',
+        'version',
+    }.difference(set(commands))
 
 
 @all_runners