Allow meta tags for custom open graph namespace

This commit allows support for custom meta tag namespaces, as required by facebook open graph for custom object definitions. 

```ruby
    set_meta_tags :open_graph => {
      title:        "My Title"
      type:         "my_custom_namespace:deal",
      url:          the_url
    }
    # new pull request code allows for this:
    set_meta_tags fb_namespace: "my_custom_namespace"
    set_meta_tags "my_custom_namespace" => {
      brand: "Brand Name",
      image: "image_path"
    }

```

This outputs:
```html
<meta content="Brand Name" property="my_custom_namespace:brand" />
<meta content="image_path" property="my_custom_namespace:image" />
```

A small modification for greatly enhanced functionality.