feature request/suggestion: add a way to add identifier to outgoing transaction

[ha1331]

The issue I'm trying to solve is when one does wallet.pay(....) you get the hash of the transaction back (this by the way is not documented at api docs, I figured this by looking at the source code), but if something goes wrong after I do .pay(...) but before I get the response, I don't have a way to know if that call made to your api, if the transaction was sent to the network or if it errored. If there was a way to supply an identifier to wallet.pay(), I could always use that to recognize the transaction when webhook fires. There might be a way to do this as it is now, but I'm not aware of it.