View Key JSON API
Inspired by https://twitter.com/btcven/status/1008935633778348032

> Does @MoneroVision have an API where the view key could be passed/used to be able to check if a payment has arrived to a Monero address?

For MoneroVision.com, it would be useful for us if onion-monero-blockchain-explorer had a JSON API where passing a viewkey could return all outputs associated with the viewkey.

@moneroexamples I think open-monero has similar functionality, so perhaps implementation could be as simple as porting the logic over.

Happy to help develop the JSON API spec if requested!
