Adds a replacer rule. For the parameters: desc is a user friendly description, enabled is true or false, matchType is one of [REQ_HEADER, REQ_HEADER_STR, REQ_BODY_STR, RESP_HEADER, RESP_HEADER_STR, RESP_BODY_STR], matchRegex should be true if the matchString should be treated as a regex otherwise false, matchString is the string that will be matched against, replacement is the replacement string, initiators may be blank (for all initiators) or a comma separated list of integers as defined in Request Initiator Constants