Add command for transferring ownership & key backup

[joehand]

Adds a dat keys command a two subcommands: dat keys import and dat keys export.

• dat keys - prints out dat key and discovery key
• dat keys export - prints out secret key (does not remove ownership from existing dat)
• dat keys import - prompts user for secret key to make a dat writable.

These are still somewhat advanced key features but will at least make it possible to backup or transfer ownership of a dat.

(#827)

[ralphtheninja]

LGTM! Really nicely implemented.

[okdistribute]

Looks awesome! Great job

[ralphtheninja]

👍

[joehand]

FYI: Waiting to fix tests before merge. Thinking that holepunchto/hypercore#109 is causing the test errors but gonna debug a bit more when mafintosh gets into town.

[austinfrey]

this is documented in the README but I don't believe it's listed in the initial dat --help printout from the command line. dat keys --help will give details though. this could very well be intentional, but thought I'd mention it.

[joehand]

Thanks, yes it is a bit intentional. Importing/Exporting via dat keys still has the potential to corrupt a dat with incorrect use. We'll leave it for more advanced use cases until the multiwriter lands to avoid that.