Allow recursive variable expansion

[shawrkbait]

This change allows recursive variable expansion.

For example: we can choose from a list of questions:

var Sentencer = require('sentencer');
var randy = require('randy');

var questions = ["what is {{ a_noun }}", "why is {{ a_noun }} so {{ adjective }}"];

Sentencer.configure({
  actions: {
    question: function() {
      return randy.choice(questions);
    }
  }
});

console.log( Sentencer.make("Question 1: {{ question }}");