Add as_dict method to Document.

[edkrueger]

• Add as_dict method to Document.
• Add test.
• Update docstring in extract_metadata to fix docs.
• Fixes extract_metadata() doesn't return dict, but documentation says it does #298.

[codecov-commenter]

Codecov Report

Merging #306 (59394f3) into master (6d8fb8d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master     #306   +/-   ##
=======================================
  Coverage   96.01%   96.02%           
=======================================
  Files          21       21           
  Lines        3240     3242    +2     
=======================================
+ Hits         3111     3113    +2     
  Misses        129      129           

Impacted Files	Coverage Δ
trafilatura/metadata.py	98.41% <100.00%> (+0.01%)	⬆️

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