expose error_handler type

the value needs to be propogated up the call stack to set for the us  er
so it's easier to name it so that we can refer to it lat              er
allow users to on_handler_error
update todo
added full splats
dont use module alias if not needed
make ** convert to FullSplat

only allow in last position however
change invalid_arg to assertion

we are validating the path on construction anyway
cleanup pattern matching in match_url
update version to 0.6.3