Add optional `expected_version` argument to `iotile release`

For releasing from a CI system that includes the version number in an environment variable or tag, there should be a way to pass that into `iotile release` so that it verifies that the version it will release is what you expect and fails otherwise.

For example:

```
$ iotile release --expected-version=$CI_VERSION_STRING
```

This would remove any potential mistakes releasing the wrong version.