Suggested addition for the signmessage command

[CIYAM]

I regularly use "signrawtransaction" with the additional "private key(s)" arg in order to *not* have to put the private key in a wallet when doing offline signing - but if I want to sign a message *offline* then I am forced to first import the private key into the wallet.

It would be nice if we could add an extra arg to "signmessage" that is a private key (so there is no need to import it into the wallet).