[WIP] Extract opengraph from body as well

[lopuhin]

Normally opengraph <meta property=".." content=".."> tags are in the head, but having them in the body is also surprisingly common - in our internal article dataset they are present in body on 5% of all pages (out of all pages with such tags anywhere on the page), and on 12% for products.

One such example is https://www.reuters.com/article/us-health-coronavirus-apple/coronavirus-case-at-apples-irish-hq-trinity-college-goes-online-idUSKBN20X1QT - so it's even on a popular website.

TODO:

• add tests
• double-check what is happening with namespaces

[codecov]

Codecov Report

Merging #129 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #129   +/-   ##
=======================================
  Coverage   87.78%   87.78%           
=======================================
  Files          11       11           
  Lines         475      475           
  Branches      103      103           
=======================================
  Hits          417      417           
  Misses         52       52           
  Partials        6        6           

Impacted Files	Coverage Δ
extruct/opengraph.py	100.00% <100.00%> (ø)

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