Make `quiet` apply to all logging.

With #228 we got rid of the need to forward a whole slew of arguments to all process running methods. We have still have this situation with the `quiet` arguments though. It is useful in all process running arguments, but it's currently missing in several places. Adding `quiet` parameters everywhere and forwarding is what we we're trying to get away from with `RunOptions`. We could add `quiet` to `RunOptions`, but I'm not sure that's the right place since it's not really specific to process running, but rather to `log` suppression. I think it'd be nice to suppress `log` for arbitrary blocks of code:

``` dart
var output = quiet(() => run(...));
log(processOutput(output));
```

it would set an internal log level, which could also be used by #101.

We'd want to deprecate any existing `quiet` parameters.