Add support for substring encryption to the authorization verification

[alexandernilsson]

Secretary verifies that the secret requested to be decrypted actually exists
in the marathon app configuration. This commit adds support for verifying
secrets that are a substring to the full value in the configuration. It
also adds a test case for the encrypted substring scenario.

[codecov-io]

Current coverage is 54.72%

Merging #5 into master will not affect coverage as of abcb5c6

@@            master      #5   diff @@
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  Files            7       7       
  Stmts          402     402       
  Branches        62      62       
  Methods          0       0       
======================================
  Hit            220     220       
  Partial         25      25       
  Missed         157     157       

Review entire Coverage Diff as of abcb5c6

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