Gruntfile.js

module.exports = function (grunt) {

  require('matchdep').filterDev('grunt-*').forEach(grunt.loadNpmTasks);
  
  grunt.initConfig({
    preprocess: {
      options: {
        context : {
          answer: 42
        }
      },
      multifile : {
        files : {
          'destination' : 'source'
        }
      }
    }
  });   

  grunt.registerTask('default', ['preprocess']);
};

package.json

{
  "name": "testing-grunt",
  "version": "0.0.0",
  "description": "Testin grunt-preprocess",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "repository": "",
  "author": "me@lazywithclass.com",
  "license": "BSD",
  "devDependencies": {
    "grunt": "~0.4.2",
    "grunt-preprocess": "~3.0.1",
    "matchdep": "~0.3.0"
  }
}

source

The answer is <!-- @echo answer -->