Add option replace

Hi, we've modified this plugin slightly to accept an option called 'replace'. I've documented it and here's how we're using it in our actual gruntfile:

```javascript
jade: {
  templates: {
    options: {
      client: true,
      namespace: 'App.Templates',
      replace: {
        'app/templates/':'',
        '/':'_'
      }
    },
    files: {
      'public/js/templates.js': [ 'app/templates/**/*.jade' ]
    }
  }
}
```

This was a simple way to override the final namespacing to accommodate our convention prior to switching to grunt. Thanks!