Machinery for versioned releases

Script to prepare and publish versioned releases.

Workflow:

```
emacs CHANGELOG.md # add entry at top for v0.1.0
git tag -a v0.1.0
git push --tags upstream
./bin/release build
./bin/release publish
```

It is not unusual for the weavedns image not to be there, if it's never been run. So `exit 1` is a bit harsh. I reckon both this and the above exits should go.