commonly useful options for XML input
can be used for option definition with haskell getopt
defines options: a_trace, a_proxy, a_use_curl, a_do_not_use_curl, a_options_curl, a_encoding,
a_issue_errors, a_do_not_issue_errors, a_parse_html, a_parse_by_mimetype, a_issue_warnings, a_do_not_issue_warnings,
a_parse_xml, a_validate, a_do_not_validate, a_canonicalize, a_do_not_canonicalize,