Add Secret Update Key

[benbjohnson]

Add a key that can be used to update the repository.

Requirements:

• Config: secret
• CLI: --secret [KEY]
• All repositories use that key.
• Update occurs at: http://hostname/:secret

[benbjohnson]

Implementation differs slightly:

• Config secret is specified per repository.
• CLI option only sets secret on default repo.
• Update occurs at http://hostname/update/:secret