Add clip command

This just exposes the underlying clip command to the user. We previously refused to do this because accidentally having secrets in the clipboard is bad. This patch addresses the objection by using the underlying --clip command to (a) put it in its own command and (b) delete the secret after 45 seconds.

This way, users have to be more intentional about sending things to the clipboard and are protected from that stuff hanging around in the clipboard forever.

Requesting review mostly because it has security implications.