add request money URI generator

[mrx23dot]

Please add a method to generate receive money links, eg: key.request_money_uri(amount_bch=0.001, label="Product one", msg="thank you")

format: bitcoincash:

[?amount=][&label=][&message=] details: https://github.com/bitcoin/bips/blob/master/bip-0021.mediawiki

This URI could be used for 3rd party QR generator library.

[ghost]

These are so easy to generate that I've never once bothered.

I'm open to adding it, just not convinced it needs it.