Add "product" namespace support

This is not a standard namespace (not mentioned in https://ogp.me/),
but it's quite common in the wild, e.g. in a product page dataset where
OG is present on ~35% of pages, this "product" namespace is present on
3.4% of pages, or ~10% of OG pages, and can often contain new
information.

Author: lopuhin

extruct/opengraph.py

@@ -11,6 +11,8 @@
     'article': 'http://ogp.me/ns/article#',
     'book': 'http://ogp.me/ns/book#',
     'profile': 'http://ogp.me/ns/profile#',
+    # non-standard but seen in the wild
+    'product': 'http://ogp.me/ns/product#',  # ~10% of product pages with OG
 }