Allow <body> attributes

This allows plugin authors to add attributes to the body tag.

This patch comes after a request by @melvincarvalho to implement `<body about="this">` as a feature to bootstrap Semantic Web and Linked Data support in Elgg.

Usage:

`elgg_register_plugin_hook_handler('output:before', 'page', 'my_body_attrs');`

Then:

```#!php
function my_body_attrs($hook, $type, $value, $params) {
    $value['body_attrs'] = array('about' => 'this');
    return $value;
}
```