Add Autokey encoder

[Jccqt]

Summary of the Change:

This pull request introduces the Autokey cipher which is a type of polyalphabetic cipher. This cipher works by choosing a key then appending it to the plaintext to form an extended key.

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.15%. Comparing base (329fc50) to head (eca9496).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
- Coverage   95.16%   95.15%   -0.02%     
==========================================
  Files         268      269       +1     
  Lines       10865    10894      +29     
  Branches     1529     1531       +2     
==========================================
+ Hits        10340    10366      +26     
- Misses        400      402       +2     
- Partials      125      126       +1     

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[siriak]

Looks good, thanks!