implement bch-wif sweeping protocol

[mr-zwets]

explenation by @jimtendo

1. A QR Code could link to a redeem page (e.g. https://stamps.cash/redeem?wallet=selene&wif=${wif})
2. The first page could prompt users to download Selene from the Play Store (or iOS equivalent).
3. The second page could present a link (e.g. bch-wif:${wif}) that could be clicked and automatically invoke Selene's Sweep function instead of instructing the user to scan a second QR Code.
   (Without a proto-handler, it means the user MUST be inside the wallet already and trigger the QR Scanner... whereas, this would allow the proto-handler to invoke the correct app, meaning we can have links that would allow sweeping like the above flow)

The proto handler would be bch-wif:${wif}