Support basic types (Boolean, Number, null) as replacements

Sometimes Boolean or Number values can be used as replacements. For example, with the following config
```js
grunt.initConfig({
  params: {
    flag: true
  },
  replace: {
    main: {
      src: [ '*.txt' ],
      dest: 'build/',
      replacements: [ {
        from: '%flag%',
        to: '<%= params.flag %>'
      } ]
    }
  }
});
```
the Grunt will pass to the plugin the following data:
```js
{
  ...
  replacements: [ {
    from: '%flag%',
    to: true // WARNING! It is Boolean, not String
  } ]
}
```
because the Grunt replaces strings like '<%= params.flag %>' with requested value as is (look at https://github.com/gruntjs/grunt/blob/v0.4.2/lib/grunt/config.js#L72).