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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