add overridable error and query views, with extensible get_query_params() and process_cdx_query()
to extend cdx for query view
add get_top_url() for adding custom top_url for frame insert
add call_with_params() for adding custom params to environ
pass headers to add_custom_params, include error message on error if available
headers: use add_header() to support multiple headers with same name
is_ajax(): check for X-Pywb-Requested-With header to make as ajax and not pass to upstream
