Integration of Domain Access and ThemeKey.

Module provides additional ThemeKey properties:
  - domain:site_name - returns current domain's site name
  - domain:id - returns current domain's id
  - domain:machine_name - returns current domain's machine name
  - domain:is_default - returns true if current domain configured as default domain
  - domain:is_alias - returns true if system:host is an alias for current domain