Allow setting prompt once readline is running

[mlasevich]

Currently there is no easy way to update the prompt once readline is called. Specifically methods setPrompt() and setRightPrompt() are not accessible in ReadLineImpl. It would be nice to be able to update the prompt without having to kill readline and start a new one.

Use case is when updating data from a secondary thread, it is sometimes nice to update the prompt(for example if prompt reflects a status that changed in the background)

Alternative is to allow variables in prompts, variables that can be updated in the background and displayed on readline.refresh()/redrawLine()

Thank you.

-M