Allow comments for LNURL-PAY invoices

Allows senders to add a short message to payments, which will be stored
as invoice memo by LND/LndHub.

app/services/lndhub.rb

@@ -49,7 +49,8 @@ class Lndhub
   def addinvoice(payload)
     invoice = post "addinvoice", {
       amt: payload[:amount],
-      description_hash: payload[:description_hash]
+      description_hash: payload[:description_hash],
+      memo: payload[:memo]
     }
 
     invoice["payment_request"]