log level formatting in functions emulator and shell

This is a follow-up of https://groups.google.com/forum/#!topic/firebase-talk/EVsq7gLkTsE

It would be nice to have better formatting in the shell when using `console.log` , `console.error`, ... For instance, colors or prefix to differentiate log levels.

It is of course possible to wrap native console behavior (I am using [loglevel](https://github.com/pimterry/loglevel) and [loglevel-plugin-prefix](https://github.com/kutuluk/loglevel-plugin-prefix)), but applying different colors does not work.

Also, one need to detect it we are running locally from `funtions:shell` or as deployed function for not duplicating log level indicator and timestamp displayed in the web-based logs viewer.

Thank you,