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 95.18%. Comparing base (329fc50) to head (3cd6860).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
+ Coverage   95.16%   95.18%   +0.01%     
==========================================
  Files         268      269       +1     
  Lines       10865    10894      +29     
  Branches     1529     1531       +2     
==========================================
+ Hits        10340    10369      +29     
  Misses        400      400              
  Partials      125      125              

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