output a confirmation after a modifying action

Still a WIP, I haven't touched the following commands yet:

```
hk sshkey-add [<public-key-file>]                                 # add ssh public key
hk transfer [-a <app>] <email>                                    # transfer app ownership to a collaborator (extra)
hk transfer-accept [-a <app>]                                     # accept an inbound app transfer (extra)
hk transfer-cancel [-a <app>]                                     # cancel an outbound app transfer (extra)
hk transfer-decline [-a <app>]                                    # decline an inbound app transfer (extra)
hk transfers [-a <app>]                                           # list existing app transfers (extra)
hk unset [-a <app>] <name>...                                     # unset env var
hk update                                                         # 
```