Add: answers hash as second argument to placeholder function

[wannabewayno]

Hi Adam,

Thanks for taking the time to make this publicly available!
Having the placeholder also receive the answers hash has helped me with a situation I needed.
I'd be more than happy to submit a pull request if you're happy for me to contribute.

Here are the proposed changes

index.js

   const choices = [
     ...this.getChoices().slice(0, this.selected),
-    this.getPlaceholder(this.selected),
+    this.getPlaceholder(this.selected, this.previousAnswers),

README.md

--**placeholder** (*String*|*Function*) The name to display as a placeholder for chosen line. If defined as a function, the first parameter will be index of currently chosen answer. (optional) default: `INSERT HERE`.
+-**placeholder** (*String*|*Function*) The name to display as a placeholder for chosen line. If defined as a function, the first parameter will be index of currently chosen answer; the second argument will be the current answers hash. (optional)
+default: `INSERT HERE`.