Make the mutt command output all matching email addresses instead of just one

This is for h2s/ppl#20. It seems like this is the way you're supposed to
implement this for integration with mutt.
Add the -i flag to the mutt command, enabling case-insensitive search

This is for h2s/ppl#21. Seems abook searches this way by default, so this would
probably have been better off as default behaviour in ppl too. Rather than
change the existing behaviour, it's going in as this flag instead. I was tempted
to go the "less is more" approach, just changing the default behaviour to this,
but part of being a well-behaved UNIXy program is being disciplined about
backwards compatibility.
Version bump for 1.16.0
Merge branch 'mutt'