Add custom logger

[sdex]

I'd like to have the ability to set a custom logger. Something like:

public interface Logger {
    void log(int level, String tag, String msg, Exception exp);
}

Shell.enableVerboseLogging = true;
Shell.logger = new Logger() {}

I can implement it and make a pull request, LMK if it's something you are willing to accept.

[watts89-cloud]

Thank you