add --show-xpub option

[dan-da]

note: I closed pr #575 and opened this instead based on develop branch instead of master because I don't know how to modify the original pr.

This pull request addresses issue #573.

User facing changes:

new flag --show-xpub "Display master xpub key for wallet and each mix level"
if flag is present:
    wallet xpub is displayed first with label: "wallet xpub:"
    mixlevel xpub is displayed before addresses with label: "xpub:".
if flag is not present, no xpub are displayed, not even for external addresses.

note: This pull request removes the display of xpub for external addresses (m/0/x/0). In my judgement, that info is mostly redundant to m/0/x xpub and clutters up the screen. A possible enhancement could be to add a verbosity level to the --show-xpub flag, in which case it could print xpub for both m/0/x/0 and m/0/x/1. Or we could simply keep the previous behavior.

note: This pull request intentionally does not address display of xpriv keys as there are potentially more privacy issues around that, and it could easily be implented with a separate flag, eg --show-xpriv.

Alternatively, the --show-xpub flag could be removed, so that xpubs are always displayed and there is is one less flag/option. ( per discussion in #496 )

Let me know your thoughts.

[coveralls]

Coverage increased (+0.02%) to 78.069% when pulling 65b8cf5 on dan-da:xpub_pull_request2 into c2ec1dd on JoinMarket-Org:develop.

[dan-da]

comments?

[AdamISZ]

I hadn't looked at this at all, so this is additional to #493 right, i.e. add xpubs for whole mixdepth and for whole wallet?

AFAIU #493 was based on the idea that you can give out an xpub for people to send funds to your wallet, while this is motivated differently. Quoting @chris-belcher from #573 "I'll leave it to someone else to think about the privacy and security aspects and decide whether it's a good idea to do this or not." The problem is I don't have time to look at everything, and this one seemed to slip through the gaps. Does anyone else have an opinion about it?

[BlinkyStitt]

Do we still need this? It looks like wallet-tool already has something to print xpubs