input(str) {
    if (!this._activePrompt) {
      return undefined;
    }
    const prompt = this._activePrompt;
    if (str === undefined) {
      return prompt.rl.line;
    }
    const width = prompt.rl.line.length;
    prompt.rl.line = str;
    const newWidth = prompt.rl.line.length;
    const diff = newWidth - width;
    prompt.rl.cursor += diff;
    const cursor = 0;
    let message = prompt.getQuestion();
    const addition = (prompt.status === 'answered') ?
      chalk.cyan(prompt.answer) :
      prompt.rl.line;
    message += addition;
    prompt.screen.render(message, {cursor: cursor});
    return this;
  }
Re-writes the input of an Inquirer prompt.
If no string is passed, it gets the current
input.

@param {String} str
@return {String}
@api public