Access metadata on parent pages

Page metadata should be available recursively. A page _/blog_ should define a key `Subpage header: Archive` and any page below that (eg _/blog/some-page_) should be able to retrieve that value:

```ruby
metadata('Subpage header', :recursive => true)
 => "Archive"
```

Also, metadata should be able to provide sensible defaults.

```ruby
metadata('Nonexistant key', :default => 'foo')
 => "foo"
```