Add a HEAD option for mutt

Something simpler than #20677 for mxcl/master. (The more complex formula can be a tap.)

At some point later, I'll resubmit some of the other especially useful Mutt patches if that seems useful. (As an example, the [confirm attachment patch](https://gist.github.com/tlvince/5741641/raw/c926ca307dc97727c2bd88a84dcb0d7ac3bb4bf5/mutt-attach.patch) seems very useful.)