Scaffold "append" operations may now append to files that were not themselves scaffolded.

When scaffolding files, an "append" operation now has two more attributes, force-append and default.

force-append: This attribute indicates that it is acceptable to append to a target file that was not scaffolded. Default is 'false'
default: This attribute specifies the relative path to a default file to use in instances where the target file was not scaffolded and does not initially exist.

Example:

      "file-mapping": {
        "[web-root]/sites/default/settings.php": {
          "append": "assets/include-local-settings.txt",
          "force-append": true,
          "default": "assets/initial-default-settings.txt"
        }
      }