Secret support

[winjer]

• Add secret = True or False flag to file resource and ensure it is honoured
• Decide on syntax for the execute command that lets you tell it where secrets are, so they aren't logged
• Review the resources so that passwords are not leaked:
  • File
  • Checkout
  • Line

[Jc2k]

Getting there with this. Need to:

• Figure out syntax for Execute so that command and commands can take passwords.
• Update Git backend to not log passwords
• Update Svn backend to not log passwords