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/Add.hs

@@ -146,7 +146,7 @@ perform file = do
 
 cleanup :: Key -> Bool -> CommandCleanup
 cleanup key hascontent = do
-	maybeShowJSON [("key", key2file key)]
+	maybeShowJSON $ JSONObject [("key", key2file key)]
 	when hascontent $
 		logStatus key InfoPresent
 	return True