Added input validation to directory and file prompts

[ramraphael]

What? Added input validation for the appRoot, app, bundleRoot, and bundleFile prompts in ribbit init.

Why? Adds an extra layer of validation to ensure that the user doesn't submit non-existent folders and files. Further checking and/or graceful error & exit messages are still needed in the main program, however.

How (to test): Install ribbit and run ribbit init in an existing React app. Entering non-existent files and folders should throw a meaningful error message.

Request for feedback: Do we need any additional validation in the initial prompt?

[mrbut]

Maybe not for the present, but a refactor would be to break up the validation function so its pieces are reusable:

const exists = (path_string) => fs.existsSync(path_string)
  ? true
  : `Entered ${type} does not exist`;

const fileType = (path_string) => {
  if (fs.lstatSync(path_string).isDirectory()) {
    return 'directory';
  }
  else if (fs.lstatSync(path_string).isFile()) {
    return 'folder';
  }
};

const pathExists = (path_string) => {
  return exists(path_string) ? true : `Entered ${fileType(path_string)} does not exist`
};

And use inquirer's answersHash to grab previous values and concatenate the paths' directories + files. Mentioned in the docs for 'validation' https://github.com/SBoudrias/Inquirer.js.

[ramraphael]

@mrbut : Updated to use Inquirer's native answers hash object! With this change, it's not apparent to me how we can further atomize this function.