Add endpoint hook support

[schloerke]

Basically copy all registerHooks to be able to be added on the endpoints.

They would be executed after router endpoints are executed at each stage.

They should be able to programmatically defined for a router in a plumber.R file. Similar to filters right now, but per endpoint.

This allows users to white list their filters per route. HUGE

#' @endpointHook NAME1
function(req, res) {
  # do things
  forward()
}

#' @get
#' @hook STAGE NAME1 NAME2 NAME3
function(req, res) {
  # do things
  forward()
}

STAGE: preroute, postroute, preserialize, or postserialize

Official names TBD.

cc @blairj09