patchbomb: introduce a 'patchbomb.confirm' option
When set to true, this option will make patchbomb always ask for confirmation
before sending the email. Confirmation is a powerful way to prevent stupid
mistakes when the sending patches.
This should let me get rid of my global alias adding
--confirm to hg email.
I know that some people may get bitten when moving from a machine with confirm
configured to a machine where it is not, but I think it is worth the risk.