Add app version and/or release stage in GitHub Issues

Implement an optional feature to add App Version and / or Release Stage from Exception as labels in GitHub Issue

```bash
coffee index.coffee --oauthToken="HIDDEN" --repo="drekinov/bugsnag-blank-test-repository" --labels="maybe last" - Pass
coffee index.coffee --oauthToken="HIDDEN" --repo="drekinov/bugsnag-blank-test-repository" --labels="" --labelAppVersion  - Pass
coffee index.coffee --oauthToken="HIDDEN" --repo="drekinov/bugsnag-blank-test-repository" --labels="" --labelReleaseStage - Pass
coffee index.coffee --oauthToken="HIDDEN" --repo="drekinov/bugsnag-blank-test-repository" --labels="" --labelAppVersion --labelReleaseStage - Pass
coffee index.coffee --oauthToken="HIDDEN" --repo="drekinov/bugsnag-blank-test-repository" --labels="bugsnag, test" --labelAppVersion --labelReleaseStage - Pass
```

You can checkout my test repository with test issues: drekinov/bugsnag-blank-test-repository#15

Resolve #41