Hide default value when show_default is False

[pfhayes]

Previously, when show_default was False on an Option, the value could still appear in the prompt. Remove this so that sensitive values can be hidden from prompts.

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [x] Add or update relevant docs, in the docs folder and in code.
• [x] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [x] Add .. versionchanged:: entries in any relevant code docs.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.

[saroad2]

Also, this PR doesn't pass pre-commit at the moment. You might want to run pre-commit run -a in order to make sure that all files are according to Click's style guideline.

[pfhayes]

Issues fixed. Added comment and also restructured code to make behavior of show_default arg to prompt more explicit