generic/output:

generic/output:
- support optional <preamble> and <postamble> which are written right
  after opening and before closing respectively. This can be used for a header
  line or an XML header.
- unescape <preamble>, <postamble>, <format>, <line_separator>, <var_separator>
  so that \t, \n, \r, \f, \v, \xnn, \nnn can be used directly (\a and \b are
  ignored; use \\ for the backslash) The long names ("carriagereturn") are still
  supported for <var_separator>, but one can just use \r, or \r\n too.
- don't abort when a chunk doesn't have a <node>. This is useful for adding
  constant chunks which consist only of a <format>, such as XML tags.