feature flag warning should be deprecation + ignore in cli

Author: rmorshea

src/idom/__main__.py

@@ -1,5 +1,9 @@
+import warnings
+
 from .cli import main
 
 
 if __name__ == "__main__":
-    main()
+    with warnings.catch_warnings():
+        warnings.simplefilter(DeprecationWarning)
+        main()

src/idom/config.py

@@ -76,5 +76,5 @@
             "by default. For more information on changes to this feature flag, see: "
             "https://github.com/idom-team/idom/issues/351"
         ),
-        UserWarning,
+        DeprecationWarning,
     )