[config] Refactoring of configurables, allowing partially configured objects.

Configurables did not allow more than one "method" option, and mixed
scenarios (options+methods+...) were sometimes flaky, forcing the user
to know what order was the right one. Now, all options work the same,
sharing the same "order" namespace.

Backward incompatible change: Options are now required by default,
unless a default is provided.

Also adds a few candies for debugging/testing, found in the
bonobo.util.inspect module.

tests/test_basics.py

@@ -5,6 +5,7 @@ import pytest
 import bonobo
 from bonobo.config.processors import ContextCurrifier
 from bonobo.constants import NOT_MODIFIED
+from bonobo.util.inspect import inspect_node
 
 
 def test_count():