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 think we should do that.

It should only require fairly minor tweaks to the Makefile, accepting a WEAVE_VERSION (or just VERSION?) env var with a sensible default and feeding that to the go build.