Ability to import existing account and / or get the account uri

[WRMSRwasTaken]

In order to use the accounturi CAA feature, the account URI is needed. I was able to extract it from /var/lib/acmed/accounts/bWNs.account.bin, but as that is a binary file, the process is rather unpleasant.

I've migrated from acme-go / lego but wasn't able to import my existing account, so I've let acmed create a new one and therefore I had to change the accounturi from all my CAA DNS records. An ability to import private keys would be a nice to have, so that some does not have to create a new account URI.