Log command output as it comes

[bdubertret]

This PR allows the webhook server to log the script output as it comes, instead of waiting for the script to finish. This is useful for our use case, where the invoked script can take a long time, for example 15 minutes to complete.

[moorereason]

Thanks for taking the time to help make webhook better. A few things:

1. I'd recommend creating an issue to discuss your enhancement request before submitting a PR.
2. PRs should be made against the development branch, not master.
3. Submit a separate PR to fix the UUID issue. That should be easier to merge.