Slightly better explanation of Prima::Application's import in ::ReadLine
Fixed twiddle() for overridden plot() in Simple.

Simple allows for the override of its plot() command. This enabled
App::Prima::REPL to very cleanly override how plotting behaves while still
fully supporting the Simple interface. Unfortunately, twiddle() was not made
aware of this possibility and caused App::Prima::REPL to hang in code that
called twiddle(). The new behavior returns immediately if plot() has been
overridden.