Add '--xml-stylesheet' option, and escape some output properly for XML.

Patch by: Yury Lebedev <yurylebedev {_AT_} mail.ru>

* cvs2cl.pl
  (XML_Stylesheet): New global variable.
  (output_header): Output the stylesheet.  Also, fix indentation.
  (output_tagdate): Escape the tag data.
  (parse_options): Accept new '--xml-stylesheet' option.
  (__DATA__): Document the new option.  Also, remove stray period in
    the documentation for the '--xml-encoding' option.