RSS_MERGER_SITELINK - According to the RSS Spec having a channel url set is required, this var helps you with that.
Other configurations
RSS_MERGER_STORAGEDIR - Location for the storage files (yes this app just stores plain JSON as everything else
seems to be overkill). Default is your configuration directory.
RSS_MERGER_CACHE_DURATION_IN_MINUTES - Cache duration, default is 15 minutes.
RSS_MERGER_PORT - Well, the port. Default is 8081.
Tech
The backend is built in Go mostly powered by Echo.
The frontend is built in ClojureScript with Reagent (React).