Enhance the HTML clipboard configuration options This release extends the `html_to_markup` filter by adding a `default=` parameter in case the filter fails, its result is the empty string, or if the filter is disabled for a certain markup language with the configuration variable `extensions.1`. The `clipboard` variable is replaced by the variables `txt_clipboard` and `html_clipbaord`. They both represent the clipboard's text content. The difference is, that the `html_clipboard` carries HTML formatting along the text, while `txt_clipboard` holds the plain text only.