Add Separator objects
Cleanup the inquirer module and change how event handler where passed context
Refactor choices normalization

Update utils.normalizeChoice to create new choices Object and to consider
separator objects.
Implement separators support on the list prompt
Add separator support to the checkbox prompt
Add separator support to the rawlist prompt
Add Separator support to the expand prompt
Add separators to lists types prompts fixture to make sure they work the same
Add README instructions to use Separators
Refactor how choices collection are handled

Added a `Choices` object containing multiple helper methods to ease the
prompt integration with the choice types.