Adds Secret wrapper for sensitive information

[koke]

I initially added this to https://github.com/wordpress-mobile/WordPressKit-iOS/pull/202 but it seems useful beyond networking, so maybe it belongs here.

Secret wraps a value that contains sensitive information to prevent accidental logging:

let password = Secret("my secret password")
print(password)             // Prints "--redacted--"
print(password.secretValue) // Prints "my secret password"