Configurable emulator logging output

When using the functions and database emulator together (with `emulator:exec`) you can get a lot of noisy logs:

```
i functions: Beginning execution of ""
i functions: Finished "" in ~1s
i functions: Beginning execution of ""
i functions: Finished "" in ~1s
i functions: Beginning execution of ""
i functions: Finished "" in ~1s
```

When combined with unit tests, the test result output can sometimes get lost in the midst of all these!

Is there anyway a flag or something (i.e. configurable options for `emulators` in `firebase.json`?) could be added to `firebase emulator:exec` to suppress this output?

I'm currently piping the output into `grep` to remove the above lines but this also removes the nice colour output of the original `firebase` command 😔 (and also doesn't properly kill the `java` emulator processes...)