Add OnBeforeTraverse and OnAfterTraverse to Invoke-TraverseDirectory

[plastikfan]

This allows the client to provide a callback to be invoked just before (OnBeforeTraverse) a directory has been traversed and then afterwards (OnAfterTraverse ). But we only invoke these, is the directory being traverse contains sub-directories; ie it is not a leaf directory (any place that invokes Get-ChildItem, should be wrapped with OnBefore/OnAfer).

eg, currently:

      # Now perform start of recursive traversal
      #
      [System.IO.DirectoryInfo[]]$directoryInfos = Get-ChildItem -Path $Path `
        -Directory | Where-Object { $Condition.InvokeReturnAsIs($_) }

      if ($directoryInfos) {
        $directoryInfos | Invoke-ForeachFsItem -Directory -Block $adapter `
          -Exchange $Exchange -Condition $Condition -Summary $Summary;
      }

would become:

      # Now perform start of recursive traversal
      #
      [System.IO.DirectoryInfo[]]$directoryInfos = Get-ChildItem -Path $Path `
        -Directory | Where-Object { $Condition.InvokeReturnAsIs($_) }

      if ($directoryInfos.Count -gt 0) {
        # Invoke-OnBeforeTraverse
      }
      if ($directoryInfos) {
        $directoryInfos | Invoke-ForeachFsItem -Directory -Block $adapter `
          -Exchange $Exchange -Condition $Condition -Summary $Summary;
      }

      if ($directoryInfos.Count -gt 0) {
        # Invoke-OnAfterTraverse
      }

[plastikfan]

This would allow a client to perform once per traversal level functionality, eg check the existence of a custom options file, or whatever else is required.