Add info about mensa and day

bot.py

@@ -55,6 +55,8 @@ async def mensa(which="zentral"):
     with urllib.request.urlopen("https://mensa.schneider-hosting.de/static/%s.%d.json" % (MENSA_URL[which], today)) as url:
         data = json.loads(url.read().decode())
 
+    BOT.say("Das Essen für %s in der %s" % (data["date"], MENSA_NAME[which]))
+
     message = ""
     for meal in data["meals"]:
         meal_line = "**%s**\n" % meal["category"]