add config option: external URLs for feeds and sitemaps

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[codecov]

Codecov Report

Merging #441 (fcb6a97) into master (2aa3197) will increase coverage by 0.00%.
The diff coverage is 87.50%.

❗ Current head fcb6a97 differs from pull request most recent head ada094f. Consider uploading reports for the commit ada094f to get more accurate results

@@           Coverage Diff           @@
##           master     #441   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
=======================================
  Files          22       22           
  Lines        3340     3342    +2     
=======================================
+ Hits         3234     3236    +2     
  Misses        106      106           

Files	Coverage Δ
trafilatura/cli_utils.py	94.64% <100.00%> (+0.02%)	⬆️
trafilatura/sitemaps.py	98.63% <100.00%> (+<0.01%)	⬆️
trafilatura/feeds.py	88.97% <75.00%> (ø)