Add HUBOT_GITHUB_ORG for issue links like repo#42

Hubot failed to provide GitHub issue links when matching `hubot-scripts#4`.

This pull requests makes him read the organization from `HUBOT_GITHUB_ORG` and combines that to create the issue link:
```sh
HUBOT_GITHUB_ORG=github
```
```
wellle: check out hubot-scripts#1377!
hubot: https://github.com/github/hubot-scripts/issue/1377
```