Add external-scripts support

This means `hubot-standup` can be put in `external-scripts.json` and everything kind of works.
FYI this now automatically includes both scripts... not sure if that was the intention, or is ideal. Anyway, [updated the documentation](https://github.com/afeld/hubot-standup/commit/a5741544bcc4df50d33541568b65faf54314de41#diff-04c6e90faac2675aa89e2176d2eec7d8L82) to reflect this.