Add multiple select (WIP need to refresh content)

[BarnabeD]

This PR add multi-select option with multiple: true option on SelectPrompt.

Need attention :

• I don't fully understand the Render class, with the stack_frame and slots. On each response, @content.reset! is called in the loop, but the prepare_content in SelectPrompt isn't recall to update the selected/unselected state of each option.
• Other point requiring validation : the ⦾ symbol for unselected options. I don't like it too much.

[fractaledmind]

Yes, the current implementation is not amenable to this feature yet, and a refactor will be needed.

The current implementation treats screen as mostly static with only the bottom few lines ("slots") as dynamic. So, prepare_content sets up the initial, static slots and the loop in ask simply reprints the static lines and then recomputes and rerenders the later dynamic slots. We will need to make the render loop take into account all slots. This will be a larger refactor, but will open up many new possibilities. You can take a stab at it if you'd like. I won't be able to focus on this until after RubyConf in November.