[IMP] component: various prop validation improvements

- add optional form for list props: ['optionalField?']
- accept undefined values for optional props
- allow declaring props with only boolean true
- throw error if extra prop is given to component

closes #223

doc/tooling.md

@@ -37,6 +37,9 @@ owl.__info__.mode = "dev";
 Note that templates compiled with the `prod` settings will not be recompiled.
 So, changing this setting is best done at startup.
 
+An important job done by the `dev` mode is to validate props for each component
+creation and update.  Also, extra props will cause an error.
+
 ## Playground
 
 The playground is an important application designed to help learning and