Extend shell so it can display quick information (flash) messages

From a usability perspective it is sometimes useful to display incremental progress of long-running operations (eg the progress of downloading a large file). Such messages are only valid for a short period of time and are frequently replaced by an updated message. This is significantly different from Roo's normal shell messages, which list actions actually completed.

It would be useful for the Shell interface to have a Shell.flash(String message) method, and allow implementations to implement this however they see fit. A default implementation in AbstractShell would simply output the flash message to the console as if it were a normal message.