Handle videos iframes and add visual indicator of external links

[benoit74]

Fix #24
Fix #42
Fix #45

See list of commits for details about changes, they are meant to be self explanatory

Nota: requires yet another change in python-scraperlib HTML rewriting engine, see openzim/python-scraperlib#209

Not ready to review because I should also work on #45 as well in same PR so that it is more obvious

[codecov]

Codecov Report

Attention: Patch coverage is 27.27273% with 40 lines in your changes missing coverage. Please review.

Project coverage is 38.44%. Comparing base (a732a8a) to head (52559e6).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
scraper/src/mindtouch2zim/processor.py	19.44%	29 Missing ⚠️
scraper/src/mindtouch2zim/vimeo.py	31.25%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   39.54%   38.44%   -1.10%     
==========================================
  Files           8        9       +1     
  Lines         612      658      +46     
  Branches       71       80       +9     
==========================================
+ Hits          242      253      +11     
- Misses        368      403      +35     
  Partials        2        2              

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[rgaudin]

LGTM ; see question