code: function getSourceTemplate(cb) {
      if (options.cache) {
        var info = self.cache[filename];
        if (info) {
          return cb(null, info.source, info.template);
        }
      }
  
      fs.readFile(filename, 'utf8', function(err, source) {
        if (err) return cb(err);
  
        var template = self.compile(source, filename);
        if (options.cache) {
          self.cache[filename] = {
            source: source,
            template: template
          };
        }
        return cb(null, source, template);
      });
    }
docs: Gets the source and compiled template for filename either from the cache
  or compiling it on the fly.