Scripts for finding accidental secrets in the repo.

This adds the following:
- A detect-secrets dependency and a related script
  (`script/detect_secrets`) to find and alert developers to secrets
  added to the code. By default, the script will search staged and new,
  unstaged files. It can optionally search only staged files.
- A whitelist, `.secrets.baseline`, that tracks instances of secrets or
  false positives already in the repo.
- Modifies `script/test` to detect secrets as part of the test suite.
- Updates to the README regarding the use of detect-secrets.

Pipfile

@@ -43,6 +43,7 @@ selenium = "*"
 honcho = "*"
 blinker = "*"
 pytest-mock = "*"
+detect-secrets = "*"
 
 [requires]
 python_version = "3.7.3"