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
```

I though it's currently unsupported anyhow... I would add a method that looks if the variable if initialised or not and returns a string either way.