feat(conf): don't load config-values for traits with ENV-VAR in metadata

+ Respect precendance rules laied in
ipython#99 (comment)

traitlets/config/configurable.py

@@ -159,7 +159,12 @@ def _load_config(self, cfg, section_names=None, traits=None):
         # hold trait notifications until after all config has been loaded
         with self.hold_trait_notifications():
             for name, config_value in my_config.items():
-                if name in traits:
+                trait = traits.get(name)
+                if trait:
+                    if trait.get_env_value() is not None:
+                        ## Env-vars take precendance over config-params.
+                        continue
+
                     if isinstance(config_value, LazyConfigValue):
                         # ConfigValue is a wrapper for using append / update on containers
                         # without having to copy the initial value