addEntity(key, value){
        if(value.indexOf("&") !== -1){
            throw new Error("Entity value can't have '&'")
        }else if(key.indexOf("&") !== -1 || key.indexOf(";") !== -1){
            throw new Error("An entity must be set without '&' and ';'. Eg. use '#xD' for '&#xD;'")
        }else if(value === "&"){
            throw new Error("An entity with value '&' is not permitted");
        }else{
            this.externalEntities[key] = value;
        }
    }
Add Entity which is not by default supported by this library
@param {string} key 
@param {string} value