Feature suggestion: Encrypted email client?

Zawinski's Law states that "Every program attempts to expand until it can read mail" - but it may seem to be useful for Keybase to be able to send messages (not necessarily to read them) from the cli client. If you add SMTP settings to Keybase, it would allow you to say for example:

`keybase message chris`

... and then it would bring up an interface where you can easily send an encrypted message to the email address associated with chris' account.

Similarly,

`keybase message chris -attach file.jpg`

... would send a message to chris' email with the file attached.

Start at the CLI client entry point and review the requested `keybase message` and `-attach` flows described in the issue. Determine the SMTP configuration, encrypted delivery, recipient email lookup, and attachment behavior required; done means the feature works end to end with documented command behavior.

Written by the indexing model from the issue text.