adds an extend API for the settings

doc/config.txt

@@ -10,8 +10,6 @@ Tiramisu is made of almost three main objects :
 - :class:`tiramisu.option.OptionDescription` is the shema, the option's structure
 - :class:`tiramisu.config.Config` which is the whole configuration entry point
 
-.. image:: config.png
-
 Accessing the `Option`'s
 -------------------------
 
@@ -47,9 +45,13 @@ object is returned, and if no `Option` has been declared in the
 
 The `Option` objects (in this case the :class:`~tiramisu.option.BoolOption`), 
 are organized into a tree into nested 
-:class:`~tiramisu.option.OptionDescription` objects. Every option has a name, 
-as does every option group. The parts of the full name of the option are 
-separated by dots: e.g. ``cfg.optgroup.optname``.
+:class:`~tiramisu.option.OptionDescription` objects. 
+
+.. image:: config.png
+
+Every option has a name, as does every option group. The parts 
+of the full name of the option are separated by dots: e.g. 
+``cfg.optgroup.optname``.
 
 Let's make the protocol of accessing a `Config`'s attribute explicit
 (because explicit is better than implicit):
@@ -362,6 +364,10 @@ read/write or read only mode::
     >>> c.cfgimpl_get_settings().remove('unknown')
     >>> print c.od1.var3
     value 
+
+Many properties can be defined at the same time on an option::
+
+    >>> c.cfgimpl_get_settings().extend(['unknown1', 'unknown2'])
     
 Properties can also be defined on an option group (that is, on an 
 :term:`option description`) let's hide a group and try to access to it::