[feature] Support payment URIs

[pacu]

Use case: Request ZEC

As a Zcash User I would like to be able to request ZEC using a Payment URI so that I can request an amount expressed in ZEC that could include a memo, an off-band message be displayed in the payee's wallet UI.

Use case: Fulfill a Payment

As Zcash use I would like my browser to recognize and fulfill a Payment Request URI