Coinbase config update?

[WillowP]

2 updates for Coinbase config could be useful:

1. The auth urls have changed, although the old ones still work (they redirect). The new urls are:

• authorize_url: 'https://login.coinbase.com/oauth2/auth'
• access_url: 'https://login.coinbase.com/oauth2/token'

2. (more important): Coinbase does not allow the word "coinbase" in their redirect url, so the default redirect url does not work. Would we consider adding redirect_url=/connect/coin/callback or similar? Honestly not too sure of the best way to handle that, but wanted to flag that the default config is not viable with coinbase Oauth config.

I'd be happy to make a PR for this just want to gauge interest first. Thanks for the great library!

[simov]

Updating the authorization URLs will be easy. As for the redirect URL, I think the best option is to list it in the Quirks section at the bottom of the main readme file - that the redirect_uri needs to be specified explicitly.

Usually the default provider name is set to be the domain name of the provider and I'm not sure if we name it coin or coinb it will make it easy for the user to guess, where the default format of the redirect_uri is documented and also the fact that specifying it explicitly overrides the default one generated internally by Grant.