[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.

bonobo/commands/convert.py

@@ -120,7 +120,8 @@ def register(parser):
     parser.add_argument(
         '--' + WRITER,
         '-w',
-        help='Choose the writer factory if it cannot be detected from extension, or if detection is wrong (use - for console pretty print).'
+        help=
+        'Choose the writer factory if it cannot be detected from extension, or if detection is wrong (use - for console pretty print).'
     )
     parser.add_argument(
         '--filter',