Support multi-tenanting

The ImageResizer architecture is designed to make multi-tenanting support easy to implement; simply implement IPlugin and ICurrentConfigProvider. A Config instance can be created from an XML node or string.

  /// <summary>
  /// Allows multi-tenancy support. The 'root' config only needs one plugin, which implements this interface.
  /// </summary>
  public interface ICurrentConfigProvider {
      /// <summary>
      /// Returns a Config instance appropriate for the current request. If null is returned, the default/root instance will be used.
      /// Implementations MUST return the same instance of Config for two identical requests. Multiple Config instances per tenant/area will cause problems.
      /// MUST be thread-safe, concurrent calls WILL ocurr, and WILL occur during initial call. 
      /// </summary>
      /// <returns></returns>
      Config GetCurrentConfig();