Update web monetization to latest API

[kookster]

The way a payment pointer is added to an RSS feed has been updated:
https://webmonetization.org/developers/rss-atom-jsonfeed/

e.g.
<link rel="monetization" href={pointer} />

and it may be the js api has been as well?
https://webmonetization.org/developers/interfaces/

There are test wallets
https://webmonetization.org/guides/test-wallet-sign-up/

and a new beta browser extension
https://webmonetization.org/supporters/get-started/