If no target is specified in sqitch revert, prompt to make sure the user wants to go all the way back to the beginning. Also add a -y option to bypass it.
Prompt before reverting.
Required for `rebase`, too. May be disabled by `-y`, or by specifying a config
variable. The `rebase` command looks at `revert.no_prompt` if
`rebase.no_prompt` does not exist.
Actually exit if the user declines to revert.
Ref Issue #49.