allow using Aeson for streaming JSON output

Keeping Text.JSON use for now, because it seems a better fit for most of
the commands, which don't use very structured JSON objects, but just output
whatever fields suites them. But this lets Aeson be used when a more
structured data type is available to serialize to JSON.

Command/AddUrl.hs

@@ -356,7 +356,7 @@ cleanup u url file key mtmp = case mtmp of
 			)
   where
 	go = do
-		maybeShowJSON [("key", key2file key)]
+		maybeShowJSON $ JSONObject [("key", key2file key)]
 		when (isJust mtmp) $
 			logStatus key InfoPresent
 		setUrlPresent u key url