code: function generateHeading() {
    var mixin = {};
    // If two char are provided, first one is used as upperline of the text.
    _.each(['==', '=', '-', '~', '\''], function(char, idx) {
      // Mustach lambda function takes no parameters and return a function
      // whose signature is data, render.
      // see https://mustache.github.io/mustache.5.html
      mixin['h' + (idx + 1)] = function() {
        return function(data, render) {
          var text = render(data);
          var length = _.last(text.split('\n')).length;
          if (char.length === 2) {
            text = repeatChar(char[1], length) + '\n' + text;
          }
          return text + '\n' + repeatChar(char[0], length);
        };
      };
    });
    return mixin;
  }
docs: Create a mixin object containing mustache lambda function for heading.
  
  The function are named h1 to h[n].
  
  @return {function} Mustache lambda