Script for tidying up old feature branches

I use feature branches a lot, and often find myself with incomplete
branches left lieing around on one of my machines (typically the one
that I didn't finish the work on).

Tidying them up involves deleting the local branch, deleting the local
"tag" for the remote branch, and removing the branch from the remote.
This script will do as little or as much of that task as is required,
depending on whether the branch or remote "tag" exists locally.