Full verbosity, helps troubleshooting

I ran `python script/bootstrap.py -v`. Still it is hard to debug when `npm install` fails inside (because of my wi-fi network problems, for example). First of all, i introduced chatty real-time function `execute_stdout`. The new function is based on `subprocess.check_call` instead of `check_output`. Then i introduced one more `verbose_mode` in `bootstrap.py` in order to add `--verbose` to `npm install`
Or we can add a `is_verbose_mode` function instead of exposing the variable directly since it is not a constant.