Feat/extract attachments

[alucryd]

Adds the ability to extract existing attachments

[codecov]

Codecov Report

❌ Patch coverage is 41.46341% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.03%. Comparing base (5c10717) to head (1287e95).
⚠️ Report is 50 commits behind head on master.

Files with missing lines	Patch %	Lines
pymkv/MKVAttachment.py	51.61%	15 Missing ⚠️
pymkv/MKVFile.py	10.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   67.25%   66.03%   -1.23%     
==========================================
  Files          10       10              
  Lines         959      995      +36     
==========================================
+ Hits          645      657      +12     
- Misses        314      338      +24     

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[GitBib]

Thank you for your contribution to the repository.
It would be good to add tests for the new functionality so we can be sure that the functions work correctly and will remain stable in the future.