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`
Three is already a `verbose_mode` in `script/lib/util.py`, I don't think we should duplicate it. How about putting it in `script/lib/config.py` and importing it from `lib.config` like `LIBCHROMIUMCONTENT_COMMIT` and `BASE_URL`?