Display log in standard output when running web target.

[jeremyfa]

Right now, we always have to manually open the developer console to display logs. It would be nice if the logs just show up on the standard output, like what happens with c++ target.

I see two ways of doing that:

1. By letting the client send its log to the node server that launched the app, but it would work only until the server timeouts, and it would only be relevant when running the app from flow command line, not for production apps (well, we may not want those logs in production apps anyway).
2. By implementing remote debugging protocols provided by the different browsers, but this would probably require more work, and browser specific implementations (I have no idea if the remote debugging protocols are similar between chrome and safari for example).

These are just some ideas. Maybe there would be a better way of addressing this, or a good reason for not doing it !