Support volume levels in XML output

I have a case where I want to write the test report to the file and have nothing report to the console. For example, I am running a server and testing against it in the same process. I want the server logs in one build step, write the test report to a file, and parse that test report in the next build step. 

If I could
```
flagpole run --all -o xml -v 0
```

I would be sooo happy 😄 🤖 