Add option to define which browser to open

Hey, I really like the feature where the browser opens after you run gRPC UI (https://github.com/fullstorydev/grpcui/issues/64). For some reason though it usually opens Safari (My default browser), but sometimes decides to open Firefox instead. I could not see a clear pattern and the command I'm using is always the same.

Would it be possible to add a flag where you can define the actual browser to not depend on the "default" value of the operating system? Would you accept a PR adding that?

Thanks!