Enable explicitly setting Roadrunner logging level

[ejulen]

Hello, recently ran into a situation at work where it would've been very useful to be able to explicitly set the Roadrunner log level without manipulating APP_ENV. Please correct me if there is another way of doing it - setting it in .rr.yaml just seems to be overridden due to the -o flag usage in StartRoadRunnerCommand.

I'd absolutely be willing to make a PR for this should you find it reasonable, with say, a new flag or something - you could still default to the current behaviour to avoid breaking backwards compatibility.

[driesvints]

Feel free to attempt a PR 👍