Inquirer

- Add a new `editor` prompt to launch an editor allowing to easily enter multiline content - or edit file content.

- Allow falsy values as option `value` in expand prompt.

- Fix `expand` prompt to correctly use the `value` key as the prompt answer.
- Fix documentation saying the `expand` prompt support the filter key.

Fixes
- Fix bug where inquirer would overwrite properties on the options passed in. Now we're making sure to not mutate your originals objects.
- On the checkbox prompt, we're now only hiding the help message once the user click the `space` key instead of any keys.
- Drop `readline2` in favor of Node core version. This might fix some edge cases some users on windows encountered.

Say hello to promises!

The whole inquirer API is now based on promises!
- The base API interface is now `inquirer.prompt(questions).then()`. There's no more callback function.
- Any async question functions is taking a promise as return value instead of requiring `this.async()`.

Breaking changes
- `filter` is now always run before `validate` 122
- `inquirer.prompt` doesn't take a callback argument anymore

Bug fixes
- Fixed list not displayed disabled options correctly 321

And various other minor fixes and cleanup to the code base.

- List prompts can now be navigated using Emacs bindings
- `expand` prompt now use the `short` question property to display inline after a choice is selected
- Bump and internal cleanup