[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/settings.py

@@ -42,6 +42,9 @@ class Setting:
     def __repr__(self):
         return '<Setting {}={!r}>'.format(self.name, self.get())
 
+    def __eq__(self, other):
+        return self.get() == other
+
     def set(self, value):
         value = self.formatter(value) if self.formatter else value
         if self.validator and not self.validator(value):