Next release!! Adding UI

Here is a proposal for next version that'll bring on more complete UI interface support to inquirer.

Right now there's to UI, the prompt UI (a prompt runner interface) and a _bottom bar_ to output logs while keeping a message (updatable) to the bottom of the screen.

Started using some stream for these new feature. This is probably something I'd like to do more in the core if I got time.

### Changelog

* `list` prompt now accept a String representing the value of the selected choice.
* `checkbox` prompt now accept an Array as default value reprensenting the checked choices values.
* Internal refactoring to build everything out of UI/Layout Object
* New `BottomBar` UI allowing to log content into a stream while keeping a bottom bar updated with status information.
* Refactor how `Separator` are handled because of Node.js "no smart" module caching. Now Separators are objects with `type=separator` property. (The `inquirer.Separator` constructor is still the recommend use, just prefer not checking `instanceof` for detection)