add __contains__

src/confthing/__init__.py

@@ -68,3 +68,9 @@ class Config:
     def __setitem__(self, path, value):
         self.set(path, value, self.clobber_subscript)
 
+    def __contains__(self, path):
+        try:
+            self.get(path, strict=True)
+            return True
+        except:
+            return False