Re-enable metadata

This is a simplistic Hash parser, but it works for basic metadata
purposes which seems better than having no metadata support at all.  The
following examples show how to set a page title using metadata.

All on one line:

```ruby
<!-- --- title: My Fantastic Page -->
```

Or split across multiple lines:

```ruby
<!-- ---
title: My Fantastic Page
-->
```

Notes:

1. Only one key/value pair per line. Multiple lines must be used for
multiple key/value pairs (though I'm not aware of support for any keys
other than "title").

2. Keys and values cannot contain embedded newlines.

3. The patch does *not* HTML-sanitize the key or value portion of the
metadata.  Presumably metadata is not solely for inclusion in HTML, so
only the user of the metadata will know whether it needs to be sanitized
in any way.